U.S. officials said Thursday they wereinvestigating a credible but unconfirmed threat that al-Qaida wasplanning to use a car bomb to target bridges or tunnels in New YorkCity or Washington to coincide with the 10th anniversary of theSept. 11 attacks, the first tip of an "active plot" around thatdate.

The Homeland Security Department said the threat is credible andspecific, but unconfirmed. The nation's terror alert level has notchanged, but raising it was under consideration Thursday night.

"There is specific, credible but unconfirmed threatinformation," said Janice Fedarcyk, the assistant director incharge of the FBI's New York division. "As we always do beforeimportant dates like the anniversary of 9/11, we will undoubtedlyget more reporting in the coming days."

New York City Mayor Michael Bloomberg told reporters that policethere were deploying additional resources around the city, but thatNew Yorkers should go about their business as usual. The city'sobservance of the attacks will go on as planned, Bloomberg said.

Police Commissioner Raymond Kelly says he's increasing the towing of illegally parked cars. He says there will be an increased focus on bridges and tunnels.

President Barack Obama was briefed on the threat informationThursday morning and directed the counterterrorism community toredouble its efforts in response to the information, a White Houseofficial said.

White House officials said there were no plans to change Obama'stravel schedule on Sunday in light of the threat. The president isscheduled to mark the 9/11 anniversary with stops at New York'sground zero, the Pentagon and Shanksville, Pa. He will also deliverremarks Sunday night at a memorial concert at the Kennedy Center inWashington.

AP wires contributed to this report.