An intelligence bulletin released by U.S. officials Friday warns of a al Qaeda potentially using a vehicle to stage a bombing attack on New York City or Washington, D.C., sometime around the 10th commemoration of Sept. 11, 2001.

U.S. officials are seeking a "handful of potential operatives" and a smaller subset that may include American citizens or legal permanent residents, national security officials tell Fox News.

An improvised explosive device attached to a car would be similar to the one used by Faisal Shahzad, who attempted to attack Times Square in 2010; such attacks may involve operatives carrying U.S. documentation.

The attacks "would be intended to cause public panic and disarray among first responders" and to show the terror group is still relevant after a year of public losses, the bulletin reads. 

Since February 2010, al Qaeda has been interested in conducting a large-scale attack and it is believed that the killing of Usama bin Laden, the bulletin reads, and the removal of several key figures in the group may contribute to a determined attack on or around the anniversary. 

More On This...

Intelligence officials said the threat is specific to car bombs at bridges and tunnels. Officials have warned about the possibility of individuals already being in the United States and sources told Fox News that they are on the lookout for two or three possible suspects, who speak English and Arabic.

Authorities who spoke to the Wall Street Journal say two of the three are believed to be U.S. citizens, while they increasingly believe one may already be in the country. The individuals were identified through flight logs and other records.

As officials ramp up their deterrence efforts, authorities are urging Americans to go about their business over the Sept. 11 weekend, but to be alert after federal authorities sent out a warning late Thursday of a "specific, credible" threat.

Vice President Joe Biden on Friday confirmed a tip, but said "there's no certitude" that anyone had traveled into the U.S. for a bomb plot. 

"The thing we are all most worried about is what they call a 'lone ranger,' a lone actor, not some extremely complicated plan like it took to take down the World Trade towers," said Biden, who appeared on three morning news shows.

President Obama was first briefed Thursday morning about the threat, and was updated through the day, source told Fox News. Officials were planning on providing additional briefings to the president on Friday. 

The White House issued a statement saying the threat was specific and credible, but also "unconfirmed."

“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 to the commemorative events marking 10 years since the Sept. 11 terror attacks, 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."

New York and Washington, D.C., law enforcement were taking extra steps to maintain vigilance in the face of the new alerts.

Sgt. Kim Schneider, a spokeswoman for the U.S. Capitol Police, said people on the Capitol Grounds or in the area of the U.S. Capitol will notice  "visible strategic security enhancements including more officers on patrol (uniformed and undercover), more police cruisers in the field, and enhanced deployments of our specialty units --" such as K9, bomb squad, Hazmat, and SWAT.

New York City Mayor Michael Bloomberg and Gov. Andrew Cuomo advised that New Yorkers should go about their business and said the city's planned observance on Sunday will go on as scheduled. 

"All New Yorkers should be cautious and aware as we prepare to commemorate the 9/11 anniversary," 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."

Police Commissioner Ray Kelly said the New York police 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.

In Washington, police are working 12-hour shifts, and Police Chief Cathy Lanier said unattended cars parked in unusual locations risked being towed.

Details provided to Fox News suggested that chatter had indicated the possible suspects were tied to Ayman Al-Zawahiri, the successor to al Qaeda's Usama bin Laden after U.S. SEALs took out bin Laden in the spring in Abbottabad, Pakistan. U.S. forces found several items in bin Laden's compound suggesting aspirational plans for an attack timed to coincide with the Sept. 11 anniversary. Authorities have received a "general description" of two or three possible suspects but don't necessarily have names, one source said. 

The person who provided the intelligence is known to the U.S. intelligence community and "has a track record," one source said.

U.S. officials had been saying for days that there is no known credible threat related to the 10th Sept. 11 since the attacks. But when asked Thursday by reporters whether that’s still the case, Homeland Security Secretary Janet Napolitano seemed to 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 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.

Click here for full coverage on the September 11th anniversary

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