WASHINGTON (AP) -- Counterterrorism officials are chasing a credible but unconfirmed tip that al-Qaida has plans to set off a car bomb in New York City or Washington around the Sept. 11 anniversary, with bridges or tunnels as potential targets. It was the first word of a possible "active plot" timed to coincide with commemoration of the terror group's attacks a decade ago.
Police in New York and Washington increased their already stepped-up staffing levels. In New York City, authorities were stopping vehicles at the 59th Street bridge Friday, causing a major traffic backup. National Guard troops and transit police carrying assault rifles watched the crowds at Penn Station.
White House officials said President Barack Obama had no plans to change his scheduled trips to New York's ground zero, the Pentagon and Shanksville, Pa., on Sunday to mark the anniversary.
This latest threat "should not surprise any of us," Secretary of State Hillary Rodham Clinton said during a speech in New York. Telling the public "is intended to enlist the millions and millions of New Yorkers and Americans to be the eyes and the ears of vigilance," she said.
Law enforcement officials were pursuing three people who may be traveling to the U.S. or who have recently entered the country, based on the detailed information received by the U.S. intelligence community late Wednesday, officials said.
Senior U.S. counterterrorism officials said Friday that authorities have a general sense of who is behind the plot and who may be tasked to carry it out, but they don't have exact identities yet. They said there is at least one U.S. citizen among the three people.
The intelligence suggested that al-Qaida planned to car bomb one of the two cities that were hit 10 years ago, in coordinated attacks that killed nearly 3,000 people.
Vice President Joe Biden said Friday that there was no confirmation that anyone had traveled into the U.S. for such a plot although the tip came from a credible source. "There's no certitude," he said.
"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 the trio of network morning TV shows Friday.
U.S. officials said the source of the terror tip indicated that al-Qaida's new leader, Ayman al-Zawahri, was involved in planning the plot. But the official also said that many in the intelligence community question that and other aspects of the source's information.
The nation's terror alert level has not changed, although raising it was under consideration Thursday night.
At the Pentagon, officials said there have been no changes to military base security levels since they were upgraded earlier in the week, before the threat information came in. And there have been no changes to the schedules of Defense Secretary Leon Panetta and Joint Chiefs Chairman Adm.