FBI Director Robert Mueller said Wednesday the FBI has placed a substantial number of people who may have ties to Al Qaeda under 24-hour surveillance throughout the country, a mission that is taxing the bureau.

Mueller's comments that people suspected of having connections with Usama bin Laden's network follow warnings made two weeks ago that another attack on the United States is likely.

"Our biggest problem is we have people we think are terrorists. They are supporters of Al Qaeda," the FBI chief said in an interview with The Washington Post published on its online edition Wednesday night. "They may have been sworn jihad [holy war], they may be here in the United States legitimately and they have committed no crime."

The FBI, which is in the midst of reorganizing, is dispatching special surveillance teams to monitor terror suspects on the ground, by air or with court-approved wiretaps, Mueller said.

He said the bureau has been "pushed, really pushed" to track all the terror suspects.

A suspect could be someone who phoned a prominent terrorist overseas or someone who passes out pro-bin Laden literature, he said.

Mueller also said the bureau has no plans for widespread infiltration of mosques to detect terrorists despite its broad new powers to monitor public places.

"We don't have a plan to go into mosques," Mueller said. "We take each investigation on its own and look at it and then what's appropriate for the investigation."

The new rules, announced last week, allow agents to attend public meetings to prevent terrorism. Old guidelines discouraging such activities were issued in the 1970s as a response to controversies about the FBI building case files against prominent Americans, including the Rev. Martin Luther King Jr.

"I want to make absolutely certain that we don't repeat those abuses of the past," Mueller said.

He said the bureau is making progress in understanding how the Sept. 11 attacks were planned. Information is being gathered from Afghanistan and overseas detainees, he said.

"A great deal of effort is being spent on exploiting the materials that have come out of Afghanistan," he said. "A great deal is being spent on the interrogation of the detainees at Guantanamo. We're all spending time and effort with our counterparts in Singapore and Malaysia and Indonesia and Saudi Arabia, Egypt and United Arab Emirates to gather whatever information we have."