FBI Director Robert S. Mueller said Thursday he believes "sleeper cells" of waiting terrorists exist even within the United States, requiring the nation to remain on extended high alert.

Mueller, meeting with reporters in the conference room outside his office at FBI headquarters, said the bureau moved "heaven and Earth" to improve security for Sunday's Super Bowl game and the upcoming winter Olympics.

"But we're still on a very high state of alert, and we will be for some time," he said.

Asked whether he believes terrorist cells exist within the U.S. borders, Mueller said there is no proof. "There may well be those in the United States. Do I know for sure? I believe there are, but I cannot say for sure." He was far more certain that such cells continue to operate overseas.

Mueller said information about possible threats to America has emerged from interviews with captured Al Qaeda soldiers and an enormous cache of documents, videotapes and other materials recovered in Afghanistan and elsewhere.

The FBI, military and U.S. intelligence agencies are keeping a painstaking inventory of captured documents and other materials, scanning them into computers and making them available to investigators on a secure digital network coordinated in Washington, said Mueller.

"We're still on a very high state of alert, and part of that comes from what you have seen as documents coming out of Afghanistan," he said.

Mueller acknowledged that the fighting in Afghanistan "has disrupted Al Qaeda" and that organization's ability to carry out major attacks. But the FBI is still frustrated in its efforts to piece together a complete picture of the events leading up to the Sept. 11 attacks against New York and Washington.

"We know half as much as we'd like about those in the attacks of Sept. 11," Mueller said.

Mueller said there were, during different periods, from two to 10 FBI agents in Kandahar interviewing captured soldiers and others but not fighting any battles. "The agents are not in the military," he said. As more soldiers are brought to the U.S. military prison in Cuba, the need for FBI agents in Afghanistan has dwindled, he said.