ASHINGTON, Oct. 5 — The Federal Bureau of Investigation is trying to make an open book of the lives of hundreds of mostly young, mostly Muslim men in the United States in the belief that Al Qaeda-trained terrorists remain in this country, awaiting instructions to attack.
Senior law enforcement officials say the surveillance campaign is being carried out by every major F.B.I. office in the country and involves 24-hour monitoring of the suspects' telephone calls, e-mail messages and Internet use, as well as scrutiny of their credit-card charges, their travel and their visits to neighborhood gathering places, including mosques.
The campaign, which has also involved efforts to recruit the suspects' friends and family members as government informers, has raised alarm from civil liberties groups and some Arab-American and Muslim leaders. The men are suspected of ties to Al Qaeda or other groups affiliated with Osama bin Laden's terrorist network.
Law enforcement officials say the surveillance program has provided vital evidence to support a string of arrests and indictments around the country since late summer — in western New York, in Detroit, in Seattle and, on Friday, in Portland, Ore. — of Americans and others accused of conspiring in terrorist cells to assist Al Qaeda.
Still, the F.B.I. has acknowledged that it has no evidence of any imminent terrorist threat posed by the so-called sleeper cells connected to Al Qaeda. Federal law enforcement officials say there is no sign of a terrorist cell operating on American soil that, in its level of commitment and training, resembles anything like the team of suicide hijackers who trained in the United States for several months before carrying out the Sept. 11 attacks.
They concede that the domestic threat posed by Qaeda cells may at times have been overstated, especially after the arrest last May of Jose Padilla, an American also known as Abdullah al-Muhajir. Justice Department officials have backed away from their initial suggestion that they had compelling evidence linking him to a plot to build an explosive radiological device known as a dirty bomb.
Still, law enforcement officials say they are convinced that at least several dozen people now under F.B.I. surveillance in the United States — with different degrees of terrorist training, and with varying degrees of loyalty to Al Qaeda — would take part in an attack if ordered, and that they represent a clear threat.
"If you look at the number of people who went through the Al Qaeda training camps, and there are literally thousands who did, it stands to reason that a certain percentage of them are in this country," said John E. Bell Jr., who retired last summer as the special agent in charge of the F.B.I.'s field office in Detroit. Much of the surveillance campaign is centered in Detroit, since the region is the home to the nation's largest population of people of Arab descent.
On Friday, Attorney General John Ashcroft announced that an investigation based in Portland had resulted in charges against six people, five of them Americans, because they were members of a "suspected terrorist cell within our borders."
The suspects were accused of conspiring to "levy war against the United States" and to contribute services to Al Qaeda. One of the suspects was accused of joining the Army Reserve to get weapons and tactical training for use in what officials said was a "jihad" against the United States. It was the latest in a series of arrests of people around the United States accused of ties to Qaeda cells.
In the New York case, six Americans of Yemeni descent from a suburb of Buffalo were accused last month of traveling to Afghanistan last year to attend a Qaeda camp.
In Detroit, four men were charged in August with membership in a "sleeper operational combat cell" tied to Al Qaeda that was preparing for attacks in the United States, Turkey and Jordan. In Seattle, James Ujaama, a local Muslim activist, was accused of providing Al Qaeda with training facilities, safe houses and computer services as part of a conspiracy to "murder and maim persons located outside the United States."
Within the Bush administration, there has been a fractious, mostly unpublicized debate over how many sleeper agents of Al Qaeda might be in the United States, and the amount of resources the F.B.I. and other law enforcement agencies should devote to their effort to ferret them out.
American counterterrorism officials have estimated that 10,000 to 20,000 young Muslims from around the world trained in Osama bin Laden's camps in Afghanistan.
Since the Sept. 11 attacks, with information gathered from abandoned Qaeda hideouts in Afghanistan and Pakistan and from captured terrorists, the officials have tried to compile the names of everyone who attended the camps. So far, the officials say, they have been able to identify and track down several hundred people around the world who trained at the camps and might be considered a threat.
Some law enforcement officials say that when they have detected Qaeda loyalists in the United States since Sept. 11, they have tended to be hapless malcontents and not disciplined terrorists.
"They are hangers-on and wannabe terrorists for the most part," said one official, adding, in reference to the leader of the Sept. 11 plot, "Mohammed Atta wouldn't have asked most of these guys to take out his trash."
But other senior officials emphasized that they were reluctant to dismiss the threat posed by these suspects, in part because they view terrorist acts like bombings as relatively easy to carry out — even by unskilled groups operating without much money or leadership.
Some Arab-American and Muslim groups have complained that the intense F.B.I. surveillance campaign, which they insist has been evident for months, has unfairly left the perception that all young men of Arab descent or the Muslim faith have some connection to terrorism.
"Young Arab men, in particular, are being treated as suspicious, possibly dangerous," said Hussein Ibish, communications director of the American-Arab Anti-Discrimination Committee. "I think there have been some really egregious instances of abuse."
But he said there was an understanding among Arab-Americans that a handful of young men of Arab descent in the United States might pose a terrorist threat, and that it was in the best interests of the community here to find and stop them. "I would be surprised if there are hundreds of them," he said. "But there could be 10, 20, 30."
In Buffalo, the F.B.I. said tips from local Muslims led to the arrest of the six men last month. Mr. Bell, the former special agent in Detroit, said that Arab and Muslim leaders in the city had "made it very clear that they were not interested in supporting terrorism, and that they were going to be the first to step forward if they learned about it."
The bureau's surveillance campaign has depended heavily on wiretaps obtained under the Foreign Intelligence Surveillance Act, which allows electronic surveillance of terrorism suspects at a far lower standard of evidence than in normal criminal cases.
Law enforcement officials said the bureau had worked closely with the National Security Agency in trying to monitor telephone calls and other communication between suspects in the United States and telephone numbers abroad that are known to be used by Al Qaeda and other terrorist groups.
The bureau's dependence on the surveillance act in the search for sleeper cells helps explain why the Justice Department has so aggressively defended its request to expand its authority under the law, passed in 1978, which has been the subject of a recent battle involving the secret court in Washington that reviews the bureau's surveillance requests.
"The terrorists don't know it, but we're listening in all the time," said a senior law enforcement official, noting that there had been extensive electronic surveillance of the six men charged near Buffalo, including reviews of e-mail messages between some of the men as they traveled in the Middle East in recent months.
In F.B.I. offices in Detroit, Newark, Los Angeles and other cities with large Arab-American and Muslim populations, officials say, the bureau is struggling to keep up with the mountain of tapes, transcripts and photographs from the surveillance.
Mr. Bell, the former director of the Detroit office, said that after Sept. 11, he doubled the number of agents assigned to work on counterterrorism — he says he is barred from providing an exact figure — and that he moved quickly to hire several Arabic-language translators to deal with the tapes and transcripts.
