Under a presidential order signed in 2002, the intelligence agency has monitored the international telephone calls and international e-mail messages of hundreds, perhaps thousands, of people inside the United States without warrants over the past three years in an effort to track possible "dirty numbers" linked to Al Qaeda, the officials said. The agency, they said, still seeks warrants to monitor entirely domestic communications.

The previously undisclosed decision to permit some eavesdropping inside the country without court approval was a major shift in American intelligence-gathering practices, particularly for the National Security Agency, whose mission is to spy on communications abroad. As a result, some officials familiar with the continuing operation have questioned whether the surveillance has stretched, if not crossed, constitutional limits on legal searches.

"This is really a sea change," said a former senior official who specializes in national security law. "It's almost a mainstay of this country that the N.S.A. only does foreign searches."

Nearly a dozen current and former officials, who were granted anonymity because of the classified nature of the program, discussed it with reporters for The New York Times because of their concerns about the operation's legality and oversight.

According to those officials and others, reservations about aspects of the program have also been expressed by Senator John D. Rockefeller IV, the West Virginia Democrat who is the vice chairman of the Senate Intelligence Committee, and a judge presiding over a secret court that oversees intelligence matters. Some of the questions about the agency's new powers led the administration to temporarily suspend the operation last year and impose more restrictions, the officials said.

The Bush administration views the operation as necessary so that the agency can move quickly to monitor communications that may disclose threats to the United States, the officials said. Defenders of the program say it has been a critical tool in helping disrupt terrorist plots and prevent attacks inside the United States.

Administration officials are confident that existing safeguards are sufficient to protect the privacy and civil liberties of Americans, the officials say. In some cases, they said, the Justice Department eventually seeks warrants if it wants to expand the eavesdropping to include communications confined within the United States. The officials said the administration had briefed Congressional leaders about the program and notified the judge in charge of the Foreign Intelligence Surveillance Court, the secret Washington court that deals with national security issues.

The White House asked The New York Times not to publish this article, arguing that it could jeopardize continuing investigations and alert would-be terrorists that they might be under scrutiny. After meeting with senior administration officials to hear their concerns, the newspaper delayed publication for a year to conduct additional reporting. Some information that administration officials argued could be useful to terrorists has been omitted.

Dealing With a New Threat

While many details about the program remain secret, officials familiar with it say the N.S.A. eavesdrops without warrants on up to 500 people in the United States at any given time. The list changes as some names are added and others dropped, so the number monitored in this country may have reached into the thousands since the program began, several officials said. Overseas, about 5,000 to 7,000 people suspected of terrorist ties are monitored at one time, according to those officials.

Several officials said the eavesdropping program had helped uncover a plot by Iyman Faris, an Ohio trucker and naturalized citizen who pleaded guilty in 2003 to supporting Al Qaeda by planning to bring down the Brooklyn Bridge with blowtorches. What appeared to be another Qaeda plot, involving fertilizer bomb attacks on British pubs and train stations, was exposed last year in part through the program, the officials said. But they said most people targeted for N.S.A. monitoring have never been charged with a crime, including an Iranian-American doctor in the South who came under suspicion because of what one official described as dubious ties to Osama bin Laden.

The eavesdropping program grew out of concerns after the Sept. 11 attacks that the nation's intelligence agencies were not poised to deal effectively with the new threat of Al Qaeda and that they were handcuffed by legal and bureaucratic restrictions better suited to peacetime than war, according to officials. In response, President Bush significantly eased limits on American intelligence and law enforcement agencies and the military.

But some of the administration's antiterrorism initiatives have provoked an outcry from members of Congress, watchdog groups, immigrants and others who argue that the measures erode protections for civil liberties and intrude on Americans' privacy.

Opponents have challenged provisions of the USA Patriot Act, the focus of contentious debate on Capitol Hill this week, that expand domestic surveillance by giving the Federal Bureau of Investigation more power to collect information like library lending lists or Internet use. Military and F.B.I. officials have drawn criticism for monitoring what were largely peaceful antiwar protests. The Pentagon and the Department of Homeland Security were forced to retreat on plans to use public and private databases to hunt for possible terrorists. And last year, the Supreme Court rejected the administration's claim that those labeled "enemy combatants" were not entitled to judicial review of their open-ended detention.

Mr. Bush's executive order allowing some warrantless eavesdropping on those inside the United States - including American citizens, permanent legal residents, tourists and other foreigners - is based on classified legal opinions that assert that the president has broad powers to order such searches, derived in part from the September 2001 Congressional resolution authorizing him to wage war on Al Qaeda and other terrorist groups, according to the officials familiar with the N.S.A. operation.

The National Security Agency, which is based at Fort Meade, Md., is the nation's largest and most secretive intelligence agency, so intent on remaining out of public view that it has long been nicknamed "No Such Agency." It breaks codes and maintains listening posts around the world to eavesdrop on foreign governments, diplomats and trade negotiators as well as drug lords and terrorists. But the agency ordinarily operates under tight restrictions on any spying on Americans, even if they are overseas, or disseminating information about them.

What the agency calls a "special collection program" began soon after the Sept. 11 attacks, as it looked for new tools to attack terrorism. The program accelerated in early 2002 after the Central Intelligence Agency started capturing top Qaeda operatives overseas, including Abu Zubaydah, who was arrested in Pakistan in March 2002. The C.I.A. seized the terrorists' computers, cellphones and personal phone directories, said the officials familiar with the program. The N.S.A. surveillance was intended to exploit those numbers and addresses as quickly as possible, they said.

In addition to eavesdropping on those numbers and reading e-mail messages to and from the Qaeda figures, the N.S.A. began monitoring others linked to them, creating an expanding chain. While most of the numbers and addresses were overseas, hundreds were in the United States, the officials said.

Under the agency's longstanding rules, the N.S.A. can target for interception phone calls or e-mail messages on foreign soil, even if the recipients of those communications are in the United States. Usually, though, the government can only target phones and e-mail messages in the United States by first obtaining a court order from the Foreign Intelligence Surveillance Court, which holds its closed sessions at the Justice Department.

Traditionally, the F.B.I., not the N.S.A., seeks such warrants and conducts most domestic eavesdropping. Until the new program began, the N.S.A. typically limited its domestic surveillance to foreign embassies and missions in Washington, New York and other cities, and obtained court orders to do so.

Since 2002, the agency has been conducting some warrantless eavesdropping on people in the United States who are linked, even if indirectly, to suspected terrorists through the chain of phone numbers and e-mail addresses, according to several officials who know of the operation. Under the special program, the agency monitors their international communications, the officials said. The agency, for example, can target phone calls from someone in New York to someone in Afghanistan.

Warrants are still required for eavesdropping on entirely domestic-to-domestic communications, those officials say, meaning that calls from that New Yorker to someone in California could not be monitored without first going to the Federal Intelligence Surveillance Court.

A White House Briefing

After the special program started, Congressional leaders from both political parties were brought to Vice President Dick Cheney's office in the White House. The leaders, who included the chairmen and ranking members of the Senate and House intelligence committees, learned of the N.S.A. operation from Mr. Cheney, Lt. Gen. Michael V. Hayden of the Air Force, who was then the agency's director and is now a full general and the principal deputy director of national intelligence, and George J. Tenet, then the director of the C.I.A., officials said.

It is not clear how much the members of Congress were told about the presidential order and the eavesdropping program. Some of them declined to comment about the matter, while others did not return phone calls.

Later briefings were held for members of Congress as they assumed leadership roles on the intelligence committees, officials familiar with the program said. After a 2003 briefing, Senator Rockefeller, the West Virginia Democrat who became vice chairman of the Senate Intelligence Committee that year, wrote a letter to Mr. Cheney expressing concerns about the program, officials knowledgeable about the letter said. It could not be determined if he received a reply. Mr. Rockefeller declined to comment. Aside from the Congressional leaders, only a small group of people, including several cabinet members and officials at the N.S.A., the C.I.A. and the Justice Department, know of the program.

Some officials familiar with it say they consider warrantless eavesdropping inside the United States to be unlawful and possibly unconstitutional, amounting to an improper search. One government official involved in the operation said he privately complained to a Congressional official about his doubts about the program's legality. But nothing came of his inquiry. "People just looked the other way because they didn't want to know what was going on," he said.

A senior government official recalled that he was taken aback when he first learned of the operation. "My first reaction was, 'We're doing what?' " he said. While he said he eventually felt that adequate safeguards were put in place, he added that questions about the program's legitimacy were understandable.

Some of those who object to the operation argue that is unnecessary. By getting warrants through the foreign intelligence court, the N.S.A. and F.B.I. could eavesdrop on people inside the United States who might be tied to terrorist groups without skirting longstanding rules, they say.

The standard of proof required to obtain a warrant from the Foreign Intelligence Surveillance Court is generally considered lower than that required for a criminal warrant - intelligence officials only have to show probable cause that someone may be "an agent of a foreign power," which includes international terrorist groups - and the secret court has turned down only a small number of requests over the years. In 2004, according to the Justice Department, 1,754 warrants were approved. And the Foreign Intelligence Surveillance Court can grant emergency approval for wiretaps within hours, officials say.

Administration officials counter that they sometimes need to move more urgently, the officials said. Those involved in the program also said that the N.S.A.'s eavesdroppers might need to start monitoring large batches of numbers all at once, and that it would be impractical to seek permission from the Foreign Intelligence Surveillance Court first, according to the officials.

The N.S.A. domestic spying operation has stirred such controversy among some national security officials in part because of the agency's cautious culture and longstanding rules.

Widespread abuses - including eavesdropping on Vietnam War protesters and civil rights activists - by American intelligence agencies became public in the 1970's and led to passage of the Foreign Intelligence Surveillance Act, which imposed strict limits on intelligence gathering on American soil. Among other things, the law required search warrants, approved by the secret F.I.S.A. court, for wiretaps in national security cases. The agency, deeply scarred by the scandals, adopted additional rules that all but ended domestic spying on its part.

After the Sept. 11 attacks, though, the United States intelligence community was criticized for being too risk-averse. The National Security Agency was even cited by the independent 9/11 Commission for adhering to self-imposed rules that were stricter than those set by federal law.

Concerns and Revisions

Several senior government officials say that when the special operation began, there were few controls on it and little formal oversight outside the N.S.A. The agency can choose its eavesdropping targets and does not have to seek approval from Justice Department or other Bush administration officials. Some agency officials wanted nothing to do with the program, apparently fearful of participating in an illegal operation, a former senior Bush administration official said. Before the 2004 election, the official said, some N.S.A. personnel worried that the program might come under scrutiny by Congressional or criminal investigators if Senator John Kerry, the Democratic nominee, was elected president.

In mid-2004, concerns about the program expressed by national security officials, government lawyers and a judge prompted the Bush administration to suspend elements of the program and revamp it.

For the first time, the Justice Department audited the N.S.A. program, several officials said. And to provide more guidance, the Justice Department and the agency expanded and refined a checklist to follow in deciding whether probable cause existed to start monitoring someone's communications, several officials said.

A complaint from Judge Colleen Kollar-Kotelly, the federal judge who oversees the Federal Intelligence Surveillance Court, helped spur the suspension, officials said. The judge questioned whether information obtained under the N.S.A. program was being improperly used as the basis for F.I.S.A. wiretap warrant requests from the Justice Department, according to senior government officials. While not knowing all the details of the exchange, several government lawyers said there appeared to be concerns that the Justice Department, by trying to shield the existence of the N.S.A. program, was in danger of misleading the court about the origins of the information cited to justify the warrants.

One official familiar with the episode said the judge insisted to Justice Department lawyers at one point that any material gathered under the special N.S.A. program not be used in seeking wiretap warrants from her court. Judge Kollar-Kotelly did not return calls for comment.

A related issue arose in a case in which the F.B.I. was monitoring the communications of a terrorist suspect under a F.I.S.A.-approved warrant, even though the National Security Agency was already conducting warrantless eavesdropping.

According to officials, F.B.I. surveillance of Mr. Faris, the Brooklyn Bridge plotter, was dropped for a short time because of technical problems. At the time, senior Justice Department officials worried what would happen if the N.S.A. picked up information that needed to be presented in court. The government would then either have to disclose the N.S.A. program or mislead a criminal court about how it had gotten the information.

Several national security officials say the powers granted the N.S.A. by President Bush go far beyond the expanded counterterrorism powers granted by Congress under the USA Patriot Act, which is up for renewal. The House on Wednesday approved a plan to reauthorize crucial parts of the law. But final passage has been delayed under the threat of a Senate filibuster because of concerns from both parties over possible intrusions on Americans' civil liberties and privacy.

Under the act, law enforcement and intelligence officials are still required to seek a F.I.S.A. warrant every time they want to eavesdrop within the United States. A recent agreement reached by Republican leaders and the Bush administration would modify the standard for F.B.I. wiretap warrants, requiring, for instance, a description of a specific target. Critics say the bar would remain too low to prevent abuses.

Bush administration officials argue that the civil liberties concerns are unfounded, and they say pointedly that the Patriot Act has not freed the N.S.A. to target Americans. "Nothing could be further from the truth," wrote John Yoo, a former official in the Justice Department's Office of Legal Counsel, and his co-author in a Wall Street Journal opinion article in December 2003. Mr. Yoo worked on a classified legal opinion on the N.S.A.'s domestic eavesdropping program.

At an April hearing on the Patriot Act renewal, Senator Barbara A. Mikulski, Democrat of Maryland, asked Attorney General Alberto R. Gonzales and Robert S. Mueller III, the director of the F.B.I., "Can the National Security Agency, the great electronic snooper, spy on the American people?"

"Generally," Mr. Mueller said, "I would say generally, they are not allowed to spy or to gather information on American citizens."

President Bush did not ask Congress to include provisions for the N.S.A. domestic surveillance program as part of the Patriot Act and has not sought any other laws to authorize the operation. Bush administration lawyers argued that such new laws were unnecessary, because they believed that the Congressional resolution on the campaign against terrorism provided ample authorization, officials said.

The Legal Line Shifts

Seeking Congressional approval was also viewed as politically risky because the proposal would be certain to face intense opposition on civil liberties grounds. The administration also feared that by publicly disclosing the existence of the operation, its usefulness in tracking terrorists would end, officials said.

The legal opinions that support the N.S.A. operation remain classified, but they appear to have followed private discussions among senior administration lawyers and other officials about the need to pursue aggressive strategies that once may have been seen as crossing a legal line, according to senior officials who participated in the discussions.

For example, just days after the Sept. 11, 2001, attacks on New York and the Pentagon, Mr. Yoo, the Justice Department lawyer, wrote an internal memorandum that argued that the government might use "electronic surveillance techniques and equipment that are more powerful and sophisticated than those available to law enforcement agencies in order to intercept telephonic communications and observe the movement of persons but without obtaining warrants for such uses."

Mr. Yoo noted that while such actions could raise constitutional issues, in the face of devastating terrorist attacks "the government may be justified in taking measures which in less troubled conditions could be seen as infringements of individual liberties."

The next year, Justice Department lawyers disclosed their thinking on the issue of warrantless wiretaps in national security cases in a little-noticed brief in an unrelated court case. In that 2002 brief, the government said that "the Constitution vests in the President inherent authority to conduct warrantless intelligence surveillance (electronic or otherwise) of foreign powers or their agents, and Congress cannot by statute extinguish that constitutional authority."

Administration officials were also encouraged by a November 2002 appeals court decision in an unrelated matter. The decision by the Foreign Intelligence Surveillance Court of Review, which sided with the administration in dismantling a bureaucratic "wall" limiting cooperation between prosecutors and intelligence officers, cited "the president's inherent constitutional authority to conduct warrantless foreign intelligence surveillance."

But the same court suggested that national security interests should not be grounds "to jettison the Fourth Amendment requirements" protecting the rights of Americans against undue searches. The dividing line, the court acknowledged, "is a very difficult one to administer."

Barclay Walsh contributed research for this article.

Securing top-secret surveillance warrants from a special court after 9/11 was proving to be hugely problematic for the Justice Department, and led a senate committee to issue an extraordinary report more than two years ago criticizing federal law enforcement officials for failing to properly follow routine guidelines in their efforts to obtain warrants for eavesdropping on Americans suspected of having ties to terrorists.

The Senate Judiciary Committee report issued in February 2003 may help explain why President Bush authorized the National Security Agency to spy on Americans without seeking prior approval from the Foreign Intelligence Surveillance Court, which for more than two decades has handled domestic spying activities.

The report singled out the FBI, and said the bureau's agents, whose job it is to obtain the surveillance warrants from the special court to collect intelligence information in the fight against terrorism, were inadequately trained in important aspects of not only the procedures to obtain warrants to spy on Americans under the Foreign Intelligence Surveillance Act (FISA), "but also fundamental aspects of criminal law."

The recent discovery of the NSA surveillance program caused a backlash against the administration by the legal community and led a judge who sits on the Foreign Intelligence Surveillance Court to resign in protest two weeks ago. The surveillance court, established by Congress in 1978 to grant warrants in terrorism and espionage cases, said it wants the Bush administration to explain why it bypassed the court and ordered eavesdropping without warrants.

Details in the 2003 senate committee report may offer an explanation. The report cited numerous problems associated with the way some officials in the Bush administration interpreted the FISA law, found a "breakdown of communication among all those involved in the FISA application process," and noted that "most disturbing is the lack of accountability that has permeated the entire application procedure."

"In fact, the bureaucratic hurdles erected by Headquarters (and DOJ) not only hindered investigations but contributed to inaccurate information being presented to the FISA Court, eroding the trust in the FBI of the special court that is key to the government's enforcement efforts in national security investigations," the report states.

President Bush and Attorney General Alberto Gonzales have said over the past few weeks that the court process was cumbersome. Still, since 9/11, the administration requested and received approval for more than 5,000 special warrants to monitor personal email accounts and conduct top-secret wiretaps of people believed to be al-Qaeda associates, according to public documents contradicting the president and attorney general's claims that the court moved too slowly in some cases.

Bush tried to explain the reasons the administration may seek approval from the special court to eavesdrop on a suspected terrorist and why, in some cases, the NSA conducts its own surveillance absent a warrant. At a December 19 press conference at the White House, he said the administration still seeks FISA warrants "for long-term monitoring," but needs the flexibility of the NSA program.

Bush said, "This is a different era, a different war.... People are changing phone numbers and phone calls, and they're moving quick. And we've got to be able to detect and prevent ... it requires quick action."

But the surveillance court has rejected just five of the nearly 19,000 requests for warrants it has received since 1979 and the warrants can be applied retroactively, meaning that the administration can begin a domestic spy operation and take up to 15 days to file a warrant request with the court.

President Bush says he has the legal authority to authorize the National Security Agency to continue eavesdropping on citizens and monitoring emails without judicial oversight, but many Democratic and Republican lawmakers are questioning whether the president violated the law in doing so.

The surveillance court has also questioned the legality of Bush's actions. The Justice Department's attempts to broaden the FBI's spying abilities after 9/11 became (such) a major concern for the surveillance court that in May 2002 it secretly ordered Attorney General John Ashcroft to scale back the plans to expand the FBI's powers because it infringed on civil liberties, according to a May 17, 2002 Foreign Intelligence Surveillance Court document.

After the Patriot Act was first approved in 2001 and a key 2002 court decision dismantled the legal wall separating the FBI's criminal and intelligence probes, the sharing of information became easier and the use of FISA warrants increased.

Ashcroft is credited with breaking down the wall that former Attorney General Janet Reno had erected in the mid-1990s that separated intelligence-gathering investigations and criminal probes to safeguard against unnecessary invasion of privacy. Federal investigators were incensed by Reno's plan, which said that intelligence agents cannot share information with criminal prosecutors, who have to meet higher legal standards to be granted warrants to conduct wiretaps and searches.

But in March 2002, Ashcroft presented a plan to the FISA court that would allow criminal prosecutors to participate in intelligence operations in the fight against terrorism. The May 17, 2002 surveillance court ruling reined Ashcroft in, and said that he overstepped his authority by loosening the rules governing intelligence gathering. Specifically, the court said Ashcroft's plans "are NOT reasonably designed" to safeguard privacy rights.

"The 2002 procedures appear to be designed to amend the law and substitute the FISA [i.e. the less demanding intelligence surveillance standards] for Title III electronic surveillances [i.e. the more demanding law enforcement standards]. This may be because the government is unable to meet the substantive requirements of these law enforcement tools, or because their administrative burdens are too onerous," the court document says.

The Senate Judiciary Committee also had concerns. The committee met privately with Ashcroft and other Justice Department officials after the Patriot Act was signed into law by President Bush on October 26, 2001. That's when Ashcroft had started to press Congress to make additional changes to FISA requirements, including changing the definition of "foreign power" to include "individual, non-U.S. persons engaged in international terrorism."

"DOJ explained that this proposal was to address the threat posed by a single foreign terrorist without an obvious tie to another person, group, or state overseas. Yet, when asked to 'provide this Committee with information about specific cases that support your claim to need such broad new powers,' DOJ was silent in its response and named no specific cases showing such a need, nor did it say that it could provide such specificity even in a classified setting," the Senate Judiciary Committee report states.

"In short, DOJ sought more power but was either unwilling or unable to provide an example as to why," the report added.

Part of the reason the FISA court refused to allow the Justice Department to expand its intelligence gathering operations goes back to the Clinton administration. In the court's May 17, 2002 opinion, it said there were more than 75 cases where "FISA applications related to major terrorist attacks directed against the United States" contained errors or false information.

Ashcroft appealed the decision. In November 2002, the court of review reversed the FISA court's decision and granted Ashcroft the broad powers he had originally sought. Still, even with the additional spying powers that now made it even easier to obtain surveillance warrants, President Bush continued to end-run the system and use the NSA to spy on Americans.

This past June, the Justice Department once again sought to broaden the scope of the Patriot Act by giving the FBI even more powers, specifically allowing the agency to bypass the FISA court if it uncovered an immediate threat to national security, which is exactly what President Bush said was the reason he had bypassed the FISA court and used the FBI to spy on Americans previously.

The FBI suggested under those extraordinary circumstances that Congress should amend the Patriot Act to provide the FBI with the authority to issue its own subpoenas without prior approval from a court. This would allow it to obtain documents, such as emails and phone records, from individuals who may have ties to terrorist groups. The FBI argued that requesting approval from a court such as the FISA court might result in an "unacceptable delay" and further threaten national security.

But in a June 17 report, the Senate Intelligence Committee said there was no reason to change the Patriot Act to grant the FBI more authority than it already possesses since the FBI could not produce any evidence where national security would be further threatened by a court taking too long to approve warrants or subpoenas.

"When testifying before the Committee, the FBI could not document significant past or current instances when national security investigations faltered or were hindered due to lack of an administrative subpoena authority," the report says. "The FBI argued that such a circumstance could exist in the future when immediacy might dictate moving quickly with a subpoena for records without prior judicial review. This may be true, but based on both demonstrated and anticipated need, the use of any such authority without prior review should be the exception, not the rule."

The committee report added that federal law enforcement officials should continue to seek approval for obtaining records from the FISA court because the court provides an "important check against potential abuse in the investigative process." Circumventing the court "effectively puts the court out of business," and "puts the current subpoena authority of the court in the hands of the investigators."

"This is not necessary, justified, or wise," the report states.

Jason Leopold spent two years covering California's electricity crisis as Los Angeles bureau chief of Dow Jones Newswires. Jason has spent the last year cultivating sources close to the CIA leak investigation, and is a regular contributer to t r u t h o u t.