U.S. officials are investigating a possible al-Qaeda plot to detonate a vehicle-borne bomb in Washington or New York City around Sunday’s 10th anniversary of the Sept. 11 terrorist attacks.

A handful of individuals may have entered the United States in recent days as part of the plot, which officials said originated from the tribal areas of Pakistan along the Afghan border. One of them may be a U.S. citizen.

Numerous officials familiar with the information cautioned Thursday night that while the threat is specific and worrisome, it is based on raw intelligence that is unconfirmed. Law enforcement agencies across the Eastern Seaboard were scrambling to determine how serious the danger is and to find any possible terrorist plotters.

Yet the mere prospect of an attack to coincide with such a sacred anniversary sparked jitters in New York and Washington, where President Obama was briefed Thursday morning and updated throughout the day, even as he prepared to address a joint session of Congress.

Members of Congress were also briefed on what law enforcement and intelligence officials described as the first specific and credible threat related to the anniversary of the attacks on the World Trade Center and the Pentagon. The concern was amplified by the knowledge that before he was killed in May, Osama bin Laden had seemed fixated on attacking the United States again on or around Sept. 11.

“As we know from the intelligence gathered from the [bin Laden] raid, al-Qaeda has shown an interest in important dates and anniversaries, such as 9/11,’’ said Matthew Chandler, a spokesman for the Department of Homeland Security. “In this instance, it’s accurate that there is specific, credible but unconfirmed threat information.’’

“We continue to ask the American people to remain vigilant as we head into the weekend,’’ he said.

The new intelligence came as security was already being ramped up nationwide, particularly in New York, where Obama and former president George W. Bush are scheduled to mark the anniversary on Sunday at Ground Zero inside what police call a “frozen zone.’’ Police are planning to cordon off the area for several blocks in all directions, forcing even residents to be escorted by police officers to their apartments.

With the latest news, officials vowed Thursday night to tighten security even further.

New York authorities said they would reinforce patrols across the city, paying special attention to bridges, tunnels and other transportation hubs, and use even more bomb-sniffing dogs.

“Over the next few days, we should all keep our eyes wide open,’’ Mayor Michael R. Bloomberg (I) said at a news conference. But he urged New Yorkers not to change their daily routines, vowing that he would take the subway to work Friday morning.

In Washington, Police Chief Cathy L. Lanier said the public should expect increased security measures and more stopped vehicles.

Police officials activated 12-hour shifts in response to the possible threat and will continue the extended duty indefinitely, officials said. Officers will be passing out fliers to city businesses and storefronts, advising the public to alert authorities about abandoned or suspicious vehicles or suspicious people who are loitering.

Mayor Vincent C. Gray (D) urged people to remain calm and report any suspicious activity.

On Thursday night, much more remained unknown than known. U.S. officials said there may be three people involved in the plot, but it was unclear how or when they may have entered the United States. One congressional source, who spoke on the condition of anonymity because the investigation is unfolding, said that the plot was connected to Afghanistan but that the connection remained unclear.

The one consistent theme of the intelligence was that the possible targets are Washington and New York.

Frances Fragos Townsend, the top counterterrorism official in the George W. Bush administration, said the terms used to describe the possible danger may have inadvertently sown confusion among the public. Townsend, who was briefed twice by senior government officials, said the quality of the information suggested that the threat was “plausible” but needed further corroboration.

“There are lots of things that are deemed ‘plausible’ that turn out not to be real,” Townsend said. She said the officials who are briefing policymakers were “leaning forward on their skis. But nobody gets in trouble for being forward-leaning.”

One federal law enforcement official concurred, saying that in the post-Sept. 11 era, the government always errs on the side of caution — and especially with the anniversary approaching.

“Given the dates that are coming up, nobody wants to underplay anything,’’ said the official, who spoke on the condition of anonymity because the threat information was not public. “The government is going to do everything it can to run this to the ground and assess its accuracy.’’

In the treasure-trove of digital and handwritten materials found at bin Laden’s compound in Pakistan in May, there were numerous references to the anniversary. The material also contained various inchoate ideas about how al-Qaeda might construct a terrorist operation, according to law enforcement and intelligence officials.

“The United States government has already significantly enhanced its security posture in advance of the 9/11 anniversary to protect the country against possible terrorist threats,’’ said a White House official, who spoke on the condition of anonymity because the intelligence was not public. “Nevertheless, the president directed the counterterrorism community to redouble its efforts in response to this credible but unconfirmed information.”

Staff writers Joby Warrick, Peter Finn, Clarence Williams and Colum Lynch contributed to this report.

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