Amid a credible terrorist attack threat against New York and Washington around the upcoming 10th anniversary of al-Qaida's deadly attack on the U.S., New York Mayor Michael Bloomberg rode the subway Friday morning in the effort to assure the city's 8 million residents that preparations are in place.

We don't want al-Qaida or any other organization ... to take away the freedoms without firing a shot, Bloomberg told the Associated Press, getting off at the City Hall stop in lower Manhattan near the Brooklyn Bridge.

Bloomberg, who also rode the subway after the city's transportation re-opened after being shut down for Hurricane Irene last week, urged New Yorkers Friday amid the threat to just go back to work. And leave it to the professionals.

On Thursday, U.S. officials said they were chasing down a credible but unconfirmed al-Qaida threat to use a car bomb on bridges or perhaps tunnels in New York or Washington -- both areas that were attacked almost a decade ago on Sept. 11, 2001.

We have received credible information very recently about a possible plot directed at the homeland that seems to be focused on New York and Washington, D.C., CNN reported a senior administration official told CNN Pentagon Correspondent Barbara Starr Thursday. 

New York police have said in light of the credible threat that they are beefing up security at bridges and tunnels, and setting up vehicle checkpoints. Police are also implementing bomb sweeps of parking garages and towing more illegally-parked cars, they said.

New York commuters have been told they will see a show of force at major transportation terminals, including Grand Central, Penn Station, and near the Port Authority and Times Square subway station.

Law enforcement officials are pursuing three individuals who may be traveling to the U.S. or have perhaps recently entered the country, based on the information received by intelligence officials, authorities said.

Officials suspect that al-Qaida's new leader, Ayman al-Zawahri, was involved in planning the attack.