WASHINGTON >> Bombsniffing dogs were deployed in the Washington subway, and police searched vehicles at New York’s Brooklyn Bridge on Friday as counterterrorism officials with frustratingly imprecise clues hunted for at least two men reportedly dispatched by al-Qaida to set off a car or truck bomb in New York or Washington.

Two senior American law enforcement officials said an informer in the Afghanistan-Pakistan region passed word of the plot, intended to coincide with the 10th anniversary of the Sept. 11 attacks, to U.S. intelligence officers on Wednesday. The informer said two U.S. citizens of Arab ancestry had left Afghanistan, traveled through one or more other countries and reached the United States as recently as last week.

But the informer’s information on the plot was second-or third-hand, another official said. It included only a vague physical description of the two men — one described as 5 feet tall, the other 5 foot 8 — and a first name for one, Suliman, that is common in the Middle East. The tipster also described a third conspirator, but he appeared to have traveled to Europe.

“All this information is very, very sketchy,” one of the law enforcement officials said.

While the informer was not specific about targets, officials in New York City and Washington increased scrutiny of bridges and tunnels, long considered potential targets for vehicle bombs.

The increased security came as Secretary of State Hillary Rodham Clinton and Vice President Joe Biden publicly discussed the threat, trying to strike a balance between urging vigilance and preventing panic. “There are specifics — in that sense it was credible,” Biden said on the ABC News program “Good Morning America,” “but there’s no certitude.”

The increased police presence forced drivers heading toward Manhattan on the Brooklyn Bridge to squeeze into a single lane and through a gantlet of police officers, who walked around and between the cars, singling out some for a closer look.

More bomb sweeps of parking garages were planned, ferries were to be given extra police coverage, and cars parked illegally were to be towed quickly, not just ticketed.

Officials briefed on the threat offered varying views of how serious it was, and some suggested that the strong reaction from federal and local agencies reflected heightened wariness around the anniversary. The two senior law enforcement officials, who were not authorized to speak publicly about the investigation, were in the skeptical camp.

One federal official said the informer reported that the two plotters were told that if tight security made a car or truck bomb impossible, they should try a less complicated attack that would sow panic.