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WASHINGTON – Vice President Biden said Friday that "every asset" in the U.S. counterterrorism arsenal was being tapped to evaluate a specific but unconfirmed threat to strike New York or Washington around the solemn 10th anniversary of the 9/11 attacks.

The vice president, who along with President Obama received updated briefings early Friday, said that the threat to use vehicle bombs against possible infrastructure targets is the first "credible" threat received by U.S. authorities since al-Qaeda leader Osama bin Laden was killed in May.

The threat involves information about three men who may have been sent to the U.S. to carry out attacks using vehicle bombs with the assistance of associates already in the country, two law enforcements officials said.

The officials are not authorized to comment publicly and declined to be identified.

In an interview on NBC's Today show, Biden acknowledged the elements of the threat but cautioned that it remained uncorroborated, beyond the original source in Pakistan.

He encouarged Americans to go about their routines, while urging that they remain "vigilant."

White House spokesman Jay Carney said the president and vice president were briefed by chief counterterrorsim adviser John Brennan, national security adviser Tom Donilon and deputy national security adviser Dennis McDonough.

"John Brennan has been … anticipating the need to be extra vigilant, to take extra precautions around the anniversary, because al-Qaeda — while it has lost its leader — remains a threat to Americans."

Since Thursday night when federal officials publicly acknowledged the threat, additional police have been assigned to the streets in both Washington and New York.

New York City Police Commissioner Ray Kelly said officers' shifts were being extended to maintain the increased presence. He also warned residents to expect increased bag checks at subway stations and vehicle checkpoints at strategic locations around the city.

The heavy security presence was a sharp counterweight to Mayor Michael Bloomberg's method of travel Friday: public transportation.

Like Biden, Bloomberg urged New Yorkers to go about their business as usual.

"There is no reason to change any of your routines," Bloomberg said.

The threat information, passed to U.S. officials by a source in Pakistan, has so far not triggered an elevation of the national threat level.

Even before the threat was received, federal and local authorities had dispatched thousands of additional personnel to landmarks and other sensitive sites in New York and Washington.

In New York alone, Kelly said "thousands" of additional officers, including snipers, bomb technicians, scuba divers and helicopter pilots, were being called to work for the anniversary.

Kelly said the increased staffing and other precautions were part of a plan to treat the anniversary date as if "an actual plot was underway."

With the possible threat still unresolved Friday, investigators were chasing leads beyond Washington and New York.

A report about a rash of stolen trucks in the Kansas City area prompted brief concerns until federal officials determined that all of the rental vehicles had been recovered.

Sources: USA Today and CNN

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