Several hundred military and civilian workers gathered at the Pentagon Friday for a somber 9/11 ceremony in the courtyard, near where Flight 77 slammed into the building, even as concerns were raised about another potential threat.

Defense Secretary Leon Panetta told the crowd that violent extremism remains a deadly threat, and "we are dealing with their threats as we speak."

U.S. authorities are chasing a credible, but unconfirmed tip that al-Qaida plans to set off a car bomb in New York City or Washington around the Sept. 11 anniversary.

Panetta offered no details on the latest threat. But he said the operation that took out Osama bin Laden made "a very clear point to our country and to the world, that nobody attacks this country and gets away with it."