WASHINGTON — When security cameras captured four young Britons sauntering into the London Underground before detonating their deadly backpacks last month, the chilling images raised questions about whether such homegrown sleeper terrorists could be plotting attacks in the United States.
U.S. counter-terrorism officials say there is no evidence that such would-be terrorists exist in large numbers in the United States, or that any of them are in the operational stages of a plot. And some U.S. officials and experts downplay the threat such domestic militants might pose to Americans.
But some senior authorities say there is enough anecdotal evidence to warrant concern, and suggest that whatever radicalized the British bombers could presumably also motivate Americans who have embraced Islamic extremist views expressed on websites and chat rooms, in radical mosques and elsewhere.
Terrorism investigators worry particularly about the American-born children of immigrants from countries known to harbor international terrorists or their training camps. An ability to move easily between cultures, and to travel widely on U.S. passports, would give such citizens a unique set of skills should they pursue terrorist intentions.
"These are second-generation Americans, people who grew up here, were educated here or were raised in this country and are now adopting this extremist view, and are now viewing their home country as the enemy," said Joseph Billy Jr., who heads the FBI's international counter-terrorism operations.
"You are talking about people who are actually here and living in the country and view us as the enemy," Billy said in an interview. "If the [terrorist] message is so strong that these people are willing to travel overseas and take up weapons, when are they going to be ready to cross the line?"
Efforts to identify and intercept anyone crossing that line have led to at least several ongoing domestic investigations, authorities confirmed in interviews. Some have resulted in arrests and prosecutions, and some have fallen apart or been downgraded to minor immigration violations.
Those not convinced that a significant domestic threat exists said most Muslim immigrants to the United States don't face the same degree of economic hardship and cultural isolation that their counterparts in Europe have endured for decades and that are thought to contribute to radicalization.
But others noted that several of the alleged London bombers appeared to have come from prosperous homes and had received good educations.
One London bombing suspect, Haroon Rashid Aswat, who was raised in Britain, had worked closely with a U.S.-spawned terrorist operative from Seattle, Earnest James Ujaama, in an abortive effort to establish a terrorist training camp in rural Oregon. Ujaama pleaded guilty in 2003; Aswat is an unindicted co-conspirator in the same case, authorities say.
"In general, terrorism recruiters are using the Internet and not focusing on the individual but rather a shotgun approach that reaches people from Portland, Ore., to Kuala Lumpur," said Matthew Levitt, a former FBI counter-terrorism analyst who heads terrorism studies at the Washington Institute for Near East Policy. "We're as susceptible as anyone else, if not more so. We'd be fools to think that what is happening in Western Europe doesn't affect us. In a globalized world, it certainly does."
But like other current and former authorities, Levitt conceded that it was difficult to know how many homegrown terrorists might be in the United States.
Billy, a deputy assistant FBI director, said he could not discuss the details of any ongoing investigations or the number of potential suspects.
But authorities from several U.S. agencies confirmed that the FBI was investigating several dozen suspected American militants operating in groups and alone, who had had varying degrees of contact with terrorist organizations in the Middle East, South Asia, Europe, North Africa and elsewhere overseas. And they said the potential numbers of such U.S.-born and bred extremists have expanded domestically apace with global antagonism toward the United States for its invasion and occupation of Iraq.
Some of the U.S. suspects are believed to have direct ties to Al Qaeda or its many affiliate groups, often through training at war camps in Afghanistan, Pakistan and other havens.
Other investigations focus on U.S. suspects linked to other terrorist groups from Central Asia and South Asia and to Palestinian terrorist organizations such as Hamas, Islamic Jihad and Al Aqsa Martyrs Brigade.
What has the FBI -- and state and local police -- so on edge is that there are "so many different degrees of connectivity" between such U.S. suspects and terrorist groups, according to a second senior U.S. counter-terrorism official. Some play a peripheral role but are nevertheless of serious concern because of their ability to provide financial, logistical and even operational assistance in the United States, he said.