New York and Washington on high alert

Washington DC and New York have been on heightened alert after credible but unconfirmed terror threat while the US intelligence agencies chase three suspects.

The number of police officials have been beefed up in the two American cities prior to the 10th anniversary of September 11 attacks.

US counterterrorism officials are chasing an unconfirmed Al Qaeda threat to attack New York or Washington in a plot timed to coincide with the 10th anniversary of the Sept. 11 attacks.

Officials said the intelligence came in late Wednesday night and indicated Al Qaeda planned to use car bombs to blow up bridges or tunnels in New York or Washington. The information also led intelligence officials to believe that there were three individuals involved who were either in the US or planned to travel here.

The threat is being described as specific and credible but unconfirmed.

Security has been ramped up around the country in the weeks leading up to the 10th anniversary, a date officials have long known Al Qaeda viewed as an appealing time to attack.

The US capital is increasing police presence during the 10th anniversary of the Sept. 11 terrorist attacks and following a threat of a new attack.

The move mirrors heightened security across the country as federal officials said Thursday they'd received specific and credible but unconfirmed information about a possible terrorist attack.

Homeland Security officials said they were investigating an Al Qaeda car bomb plot aimed at bridges or tunnels in New York or Washington. Officials say unattended cars parked near critical infrastructure will be towed.

Washington D.C. Police Chief Cathy Lanier says officers will be working 12-hour shifts for the near future. She said in a written statement that the scheduling changes were "part of our plan" and that "maintaining a certain sense of unpredictability is essential to the success of any security plan."

New York City is beefing up security at bridges and tunnels and commuters can expect a show of force at train stations around the city in response to a potential terror threat before the 10th anniversary of the Sept. 11 attacks.

Mayor Michael Bloomberg says New Yorkers should be vigilant. The police department "is deploying additional resources... some of which you will notice and some of which you will not," Bloomberg told reporters.

U.S. counterterrorism officials are chasing a credible but unconfirmed Al Qaeda threat to use a car bomb on bridges or tunnels in New York or Washington.

Police Commissioner Raymond Kelly says police are setting up vehicle checkpoints and doing bomb sweeps of parking garages.

City residents will see more bomb-sniffing dogs, bag checks at subways and additional officers at Grand Central Terminal and Penn Station on Friday.