'Sleeper cells' keep officials up

WASHINGTON -- National intelligence director Mike McConnell said Tuesday that U.S. authorities are worried about "sleeper cells" of would-be terrorists inside the United States and are remaining vigilant against any new attacks.

On the sixth anniversary of the attacks that killed nearly 3,000 people in New York, Washington and in western Pennsylvania, McConnell also said plots against the United States have been thwarted. But he said he can make no safety guarantees.

"We're safer but we're not safe," he said in an interview on ABC's "Good Morning America."

McConnell said that Osama bin Laden's al-Qaida terrorist network "intends to have an operation in the United States that will result in mass casualties."

"We have stopped some efforts and we must stop all efforts. We're not sure we can stop 100 percent of them," he said.

McConnell spoke as U.S. intelligence experts continued to assess the latest messages from bin Laden. In a new video released Tuesday, bin Laden urged sympathizers to join the "caravan" of martyrs and he praised one of the Sept. 11 suicide hijackers.

The intelligence czar said he could not immediately read anything substantial into bin Laden's tape.

"We look at these tapes very, very closely," he said. McConnell said that bin Laden remains a prodigious threat to America, but said the "intellectual leader" of al-Qaida is Egyptian Ayman al-Zawahiri.

The search goes on for bin Laden, believed to be hiding in the mountains of Pakistan near the Afghanistan border, but he remains elusive.

"Finding a single human being in the billions that are on the earth, that wants to remain hidden ... makes it very, very difficult," said McConnell, acknowledging that bin Laden has been "virtually enjoying a safe haven."

" ... Even if we did find him and remove him from the scene, he would be seen as a martyr," he said of bin Laden.

"We worry about sleeper cells in the United States," McConnell added. "There are al-Qaida sympathizers ... but so far we have not been able to identify them."

"The worry is that we have to maintain our vigilance," he said. "We have stopped many efforts to come into the United States, so we have been successful. But we cannot let our guard down."

McConnell also said that, so far, U.S. authorities have not been able to "identify" any groups which may have gained access to nuclear materials.