US vice president Joe Biden said on Friday that a possible car bomb plot may be at the heart of a credible but unconfirmed terror threat around the 10th anniversary of the 9/11 attacks.
"We don't have a smoking gun, but we do have talk about using a car bomb," Biden told the ABC network's "Good Morning America" show.
The vice president said the threat - on which both he and President Barack Obama were briefed - contained "specifics" that made it credible, but "there is no certitude."
Heavily armed police were on alert in and around New York City after US officials on Thursday warned of a "credible" but unconfirmed bomb threat around the 10th anniversary of the 9/11 attacks.
Federal officials in Washington, speaking on condition of anonymity, said there was a potential terrorism threat involving bomb-laden vehicles against either the capital or New York.
Biden said documents seized during the raid that killed al Qaeda founder Osama bin Laden in Pakistan "indicated he was interested in an attack on 9/11."
When asked if extremists were plotting to enter the United States with a car bomb, Biden said "we were told that that was an intention, to get people into the United States to do that."
The intelligence came from "a credible source," he said, but "we do not have confirmation" that a specific plot was underway.
The White House has been preparing for months for potential terrorism plots in the run up to the 10th anniversary of the worst terror attacks in US history.
"We've been planning on this," Biden stressed, adding that Obama met with his chief counter-terrorism advisory John Brennan four months ago and told him to "run down every lead possible related (to) 9/11."