NEW YORK - New York City Mayor Michael Bloomberg says a terror threat against the city is credible but not corroborated.
U.S. officials said Thursday they were investigating a detailed al Qaeda car bomb plot aimed at bridges or tunnels in New York or Washington.
Bloomberg says the New York Police Department is deploying additional resources to keep residents safe ahead of the 10th anniversary of the Sept. 11 terror attacks on Sunday. He says the city's 9/11 observance will go ahead as planned.
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.
Law enforcement officials were investigating three people who recently entered the U.S. The threat was received by the U.S. intelligence community late Wednesday night, officials said.
"There is specific, credible but unconfirmed threat information," said Janice Fedarcyk, the assistant director in charge of the FBI's New York division. "As we always do before important dates like the anniversary of 9/11, we will undoubtedly get more reporting in the coming days."
According to CBS News correspondent Bob Orr, intelligence officials say this threat is "specific to targets, methods and operatives," but remains unconfirmed, meaning that intelligence agencies across the government have picked up a threat stream from overseas, and that there is a full court press by the FBI, CIA and others to refute or corroborate the information.
One official tells CBS News the government decided to alert the public about this specific threat because, "there are enough specifics in the threat to make it more than 'aspirational'."
ABC News reports that at least one of the three people under suspicion is a U.S. citizen, and the others may carry green cards or U.S. passports but their immigration status was unclear, according to officials in the intelligence community.
Sources told CBS News that the threat specifically has come from Pakistan, which is home to various terror groups, including the Pakistani Taliban, the organization behind the failed Times Square bombing attempt, and many senior al Qaeda leaders.
Information gleaned from Osama bin Laden's compound in Pakistan in May indicated al Qaeda had considered attacking the United States on the 10th anniversary of the Sept. 11 attacks and other important dates.
A federal law enforcement source also told CBS News that operatives may be acting on bin Laden's wishes to time an attack to the anniversary.
One of al Qaeda's senior operations planners, Sheikh Yunis Al-Mauritani, was arrested just days ago in a Quetta suburb in an operation assisted by the CIA. His arrest may also have yielded information related to any current threats.
While the report is unconfirmed, a number of officials say there are enough elements lined up to have them concerned about this threat stream more so than the others they have seen in recent days.
A White House official told CBS News White House correspondent Mark Knoller that President Obama was briefed about the threat Thursday morning and had been updated all day.
"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 the official. "Nevertheless, the President directed the counterterrorism community to redouble its efforts in response to this credible but unconfirmed information."
White House officials said there were no plans to change Obama's travel schedule on Sunday in light of the threat. The president is scheduled to mark the 9/11 anniversary with stops at New York's ground zero, the Pentagon and Shanksville, Pa. He will also deliver remarks Sunday night at a memorial concert at the Kennedy Center in Washington.