Vice President Joe Biden said today that while intelligence on a potential terror plot against New York City or Washington, D.C., came from a credible source, it has not been confirmed and there is "no certitude" an attack has been planned.
Biden said he and President Obama had been briefed on intelligence developed by the CIA in the last 48 hours that three men may have entered the country with the intent to launch a vehicle-bomb attack on the homeland.
"We do have talk about using a car bomb. We've been told that was an intention... from a credible source," Biden said on "Good Morning America." "But we do not have confirmation of that."
"We don't have a smoking gun," he said.
The FBI and Department of Homeland Security sent a bulletin out to law enforcement agencies across the country late Thursday warning against the potential plot by al Qaeda surrounding the tenth anniversary of Sept. 11 to "avenge Osama bin Laden's death."
Although authorities said they had not yet identified the suspects, they are looking at multiple names of individuals who entered the U.S. after mid-August. The operatives are believed to have embarked on their journey to the U.S. from the tribal areas of Pakistan, according to two senior officials. One official said the route may have taken them through Dubai. The threat information was obtained recently and originated from overseas sources -- Pakistan, according to one official.
Intelligence and law enforcement officials told ABC News that at least one of the individuals is a U.S. citizen and one official said that two of the individuals may have had U.S. documentation -- whether green cards or passports was unclear.
The information on the plot was "very specific" said one official, adding, "It seems like the information has been worked for a while."
Officials told ABC News Thursday the alleged terror plot was initiated by new al Qaeda chief Ayman al-Zawahiri, Osama bin Laden's successor, who had pledged to avenge bin Laden's death earlier this year at the hands of U.S. Navy SEALs.
"As we know from the intelligence gathered from the Osama bin Laden raid, al Qaeda has shown an interest in important dates and anniversaries, such as 9/11. In this instance, it's accurate that there is specific, credible but unconfirmed threat information," Department of Homeland Security spokesman Matthew Chandler said in a statement Thursday.
But while the FBI, CIA and other federal agencies rush to identify the men linked to the potential plot, several officials in addition to Biden emphasized that the information has not been confirmed.
"Credible means this is something they've been expecting, something that could happen," former White House counter-terrorism advisor and now ABC News consultant Richard Clarke said on "GMA". "Unconfirmed means they have one source... Sometimes people make up stories and pass them on to the CIA in great detail."
In New York, police commissioner Ray Kelly announced Thursday evening police would be deploying additional bomb-sniffing dogs, radiation detection equipment and random vehicle checks beginning Friday morning. There also will be stepped up bag searches, towing of illegally parked cars and increased police presence on the ferries, Kelly said.
Initially, one official added that at least two rental trucks -- one from Penske and one from Budget -- were being sought nationwide. Those trucks have since been recovered and appear to have had no connection to the plotters.
In Zawahiri's most recent video, last month, he called for his followers to focus on the U.S.
"Hunt her down wherever you may encounter her. Hunt her down to cut what is left of her corruption's tail," Zawahiri says in the video.
"We know from Bin Laden's own handwriting he wanted to do an attack around the anniversary," said Clarke. "We know from his successor's own audio tapes and video tapes that he feels he has to prove al Qaeda is still alive by avenging bin Laden's death. And we know that this kind of technique could be relatively easily done, even by an al Qaeda that's on the ropes, even by an al Qaeda that has very few people left."
Aaron Katersky and Lee Ferran contributed to this report.