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 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.

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-Qaida by planning to bring down the Brooklyn Bridge with blowtorches. But they said most people targeted for NSA monitoring have never been charged with crimes. What the agency calls a "special collection program" began soon after the Sept. 11 attacks. The program accelerated in early 2002 after the Central Intelligence Agency started capturing top al-Qaida operatives overseas, including Abu Zubaydah, who was arrested in Pakistan in March 2002. The CIA seized the terrorists' computers, cell phones and personal phone directories, said the officials familiar with the program. The NSA surveillance was intended to exploit those numbers and addresses as quickly as possible, the officials said.

In addition to eavesdropping on those numbers and reading e-mail messages to and from the al-Qaida figures, the NSA 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 long-standing rules, the NSA 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 this country by first obtaining a court order from the Foreign Intelligence Surveillance Court, which holds its closed sessions at the Justice Department.

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.