Washington ? The possibility that terrorist “sleeper cells” are working undetected in America is near the top of worries for counterterrorism officials. This concern is brought home by evidence that seemingly ordinary young men carried out the London bombings.

Particularly unnerving is that last week’s bombings and those in Madrid last year suggest extremists already in place – in some cases, native-born citizens – can plan and execute an attack without attracting police attention.

The government raised the terror alert to high for mass transit systems after the London attacks. But officials said they have no credible, specific threats of an attack on America.

Even before the attacks on the London mass transit system, FBI Director Robert Mueller said the Madrid bombings “have heightened our concern regarding the possible role that indigenous Islamic extremists, already in the U.S., may play in future terrorist plots.”

Some experts caution that there are vast differences between Muslim communities in Europe and the United States. Still, Mueller told Congress in February, “I remain very concerned about what we are not seeing.”

The would-be attackers might al-Qaida operatives – sleeper agents who have been here for some time and are awaiting an order – or homegrown terrorists who are influenced, if not directed, by al-Qaida, Mueller said.

British investigators appear to be pursuing leads that suggest the suspected London bombers – at least three of whom are British citizens of Pakistani descent – fall into the latter category.

In his annual assessment of the terror threat, Mueller also said that some converts to Islam could be motivated to undertake attacks.

The Bush administration has moved aggressively against people suspected of training for and plotting attacks, Critics have said the charges have been overblown and unrelated to terrorism in many cases.