FBI Director Robert Mueller (MUHL'-ur) says the intelligence community continues to investigate a credible and specific threat it received last week, timed to coincide with the 10th anniversary of Sept. 11.

Since late last Wednesday, the government has been trying to confirm that operatives were inside or on their way to the U.S. with the intent to use a car bomb in either New York or Washington. Officials believe that al-Qaida leader Ayman al-Zawahri might be behind the plot.

Mueller says the threat remains unconfirmed, but the government has had hundreds of interviews with people and is pursuing leads.

He says though the anniversary passed without incident, that doesn't mean the counterterrorism community should relax.

Mueller was speaking at a Senate hearing Tuesday on the terror threat to the U.S.