Federal authorities are warning local law enforcement agencies of a potential terrorist threat involving car bombs that could coincide with the 10th anniversary of the Sept. 11 attacks, several sources told Fox News on Thursday.

The Department of Homeland Security confirmed that it had information about a "specific, credible but unconfirmed threat," and the White House said President Obama was briefed Thursday morning and has been updated throughout the day.

“The president directed the counterterrorism community to redouble its efforts in response to this credible but unconfirmed information," a White House official told Fox News.

Law enforcement officials already have been on alert for any potential threats timed for the 9/11 anniversary, but intelligence on the threat identified Thursday was described by one official as noteworthy for seeming "more than aspirational."

Another senior U.S. official with knowledge of the threat said it was "specific enough to elicit worry."

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Details trickled in throughout the evening. The potential threat is said to focus on New York City or Washington, D.C., and involves possible suspects tied to Al Qaeda, sources said. Authorities have received a "general description" of two or three possible suspects but don't necessarily have names, one source said. Authorities are trying to determine if such suspects might already be in the U.S.

The FBI and Department of Homeland Security were drafting a bulletin Thursday night to send to local law enforcement across the country.

"We have a duty to warn,” the senior U.S. official said.

The NYPD will be taking additional precautions in response to the terror threat, including increasing its patrol by a third, setting up vehicle checkpoints, and working closely with the FBI to gain any additional intelligence, Police Commissioner Kelly said in a press conference with New York Mayor Bloomberg and an FBI official.

"All New Yorkers should be cautious and aware as we prepare to commemorate the 9/11 anniversary," New York Gov. Cuomo said in a statement. "However, there is no reason to panic or allow our spirit of freedom to be dampened as we get ready to celebrate the opening of the Ground Zero site this weekend."

U.S. officials had been saying for days that there is no known credible threat related to the 9/11 anniversary. But when asked Thursday by reporters whether that’s still the case, Homeland Security Secretary Janet Napolitano seemed to at first hesitate.

"It is still the case that we don't have something that would reach that standard, but we still have lots of chatter out there. And we take every bit of that seriously and track it down," Napolitano said. "The date is important because it's 9/11. But as I said, in the intel world there's lots of chatter and we're taking it all seriously.

"Should there be something that rises to the level where I have to issue a threat advisory, we will issue a threat advisory."

One U.S. official said that the intelligence on the potential 9/11 anniversary threat came from tribal areas of Pakistan. The person who provided the intelligence is known to the U.S. intelligence community and "has a track record," as one source put it.

There were documents found inside Usama bin Laden's compound in Abbottabad, Pakistan, that mentioned aspirations to strike on the 10th anniversary of the 2001 attacks.

One Fox News source emphasized that authorities found themselves in a similar scenario on the eve of Obama's inauguration in January 2009, when intelligence suggested a specific threat against the inaugural events. Federal authorities reacted swiftly, but in the end the intelligence turned out to be bogus.

Fox News' Ed Henry, Catherine Herridge and Justin Fishel contributed to this report.