U.S. officials searched travel databases, combed airline manifests and tightened security Friday in a search for what could be a handful of al-Qaeda operatives suspected of plotting a terrorist attack on Washington or New York.

The day after the government revealed a possible plot to detonate a vehicle-borne bomb around Sunday’s 10th anniversary of the Sept. 11 attacks, investigators and the public reacted with caution and concern. New York City police set up vehicle checkpoints and posted rifle-clad officers at train stations, while in Washington, “suspicious activity” calls to authorities jumped about 25 percent.

Two U.S. officials briefed on the threat said it is based on intelligence that the CIA collected from a source in Afghanistan who has previously provided reliable information. The source contended that a handful of individuals, one of whom could be a U.S. citizen, may have entered the United States in recent days as part of the plot.

That prompted a flurry of activity Friday, as officials from the FBI, the National Counterterrorism Center and other agencies searched manifests and databases for any clues of who the suspected operatives are and where they might have traveled.

“We’re looking at travel records, times, dates that people may have traveled, passenger lists, itineraries,” said a U.S. counterterrorism official who, like other officials, spoke on the condition of anonymity because the investigation was not public. “All those things that would help to narrow down the type of person you might be looking for.”

Yet it remained unclear how serious the danger might be. Vice President Biden, in television interviews Friday morning, said there was “no certitude” that any plotters had arrived in the United States.

Officials cautioned that while the information is considered credible, it is also raw intelligence that had not been corroborated elsewhere. They characterized the search for the suspected operatives, whose names were apparently unknown and whose descriptions were vague, as a broad sweep with few specific clues.

“We only have one source,’’ New York Mayor Michael R. Bloomberg (I) told the Fox Business Network. “I don’t mean to make light of it, because we’ve ramped up our security. . . . If you ask my best guess, we will go through this weekend and Monday start a new era.’’

Added one federal law enforcement official briefed on the threat: “If they had more information, you’d see them knocking down doors and arresting people.’’

Others emphasized that Americans should not allow fear of an attack to alter their daily lives.

“We are determined not to let the specter of terrorism darken the national character that has always been America’s asset,” Secretary of State Hillary Rodham Clinton said in a policy speech marking the anniversary.

On Capitol Hill, Terrance W. Gainer, the Senate’s sergeant at arms and a member of the board of the U.S. Capitol Police, said in an e-mail to senators, aides and others that “there is no reason to change the way we each conduct our daily business.”

“Frankly, the threat is not unexpected,” he wrote, adding that Thursday’s government warnings were “well-intentioned, perhaps helpful, but not very well-coordinated.’’

Even with the caveats, people familiar with the information described it as the first specific and credible threat related to the anniversary of the attacks on the World Trade Center and the Pentagon. Amplifying the concern was the knowledge that before he was killed in May, Osama bin Laden had seemed fixated on attacking the United States again on or near Sept. 11. The treasure-trove of materials found at bin Laden’s compound in Pakistan included numerous references to the anniversary, according to law enforcement and intelligence officials.

The new intelligence came as security was already being increased nationwide, particularly in New York, where President Obama and former president George W. Bush are scheduled to mark the anniversary Sunday at Ground Zero. The White House said Friday that Obama has no plans to change his scheduled trips to Ground Zero, the Pentagon and Shanksville, Pa., to honor the Sept. 11 victims.

With the latest news, officials vowed to tighten security even further. D.C. Police Chief Cathy L. Lanier said at a news conference that officers are working 12-hour shifts citywide, with 200 to 400 additional officers per shift.

“You will see a heightened level and presence out in the community,’’ said Lanier, who urged people to “keep your eyes open.’’ The city’s heightened reports of “suspicious activity,’’ she said, included people calling about vehicles parked with someone inside.

In Montgomery County, police will be deployed in uniform and in plainclothes. They will monitor “soft targets,” such as shopping malls and government buildings, as well as individuals who live in Montgomery and could be targets, officials said.

“We are ramped up even more from what we had planned,” said Montgomery Police Chief J. Thomas Manger.

Staff writers William Branigan, Colum Lynch, Theola Labbé-DeBose, Dan Morse, Paul Kane and Joby Warrick contributed to this report.