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.