The Al Qaeda (search) presence in the United States is small and mainly involved in support for possible future attacks, but the terror network remains the most severe threat to Americans, the FBI's top counterterrorism (search) official said Thursday.

The FBI has not detected any individuals or cells that appear to be actively planning attacks like those almost two years ago that killed some 3,000 people in New York, Washington and Pennsylvania.

Instead, most of the Al Qaeda operatives in the United States are here to provide logistical support such as travel documents, recruitment and fund-raising, said Larry Mefford, who heads FBI counterterrorism and counterintelligence.

"My view is, it's very small but it does exist," Mefford said of the Al Qaeda presence. "We do have evidence of it."

Mefford told reporters that the FBI's strategy for such people is to keep them under surveillance for as long as possible to uncover their Al Qaeda contacts in the United States and overseas. At the same time, criminal violations they commit are carefully tracked so that the individuals can be arrested at a moment's notice to disrupt or prevent terrorist operations from going forward.

"That's a judgment call that we will make tactically," Mefford said.

Mefford cautioned that the FBI may not have identified all of the Al Qaeda operatives in the United States, and someone unknown could be plotting destruction similar to the attacks of Sept. 11, 2001.

"They do have the ability to change and adapt to their environment," he said. "Al Qaeda remains our No. 1 concern."

Another grave concern is the lone individual sympathetic to Muslim extremists (search) who decides to conduct a deadly operation on his own.

"It's very difficult to stop these types of attacks," Mefford said.