NEW YORK (AP) — Mayor Michael Bloomberg rode the subway Friday morning to assure commuters that the city is fully prepared for a potential terror threat before the 10th anniversary of the Sept. 11 attacks.

“We take every single threat that is credible seriously,” Bloomberg said as he stood on the platform at the City Hall subway stop in lower Manhattan. “We may feel that it’s more likely or less likely but you never want to say ‘well we didn’t think that was very likely’ so we didn’t do anything.”

U.S. officials said Thursday that they were chasing a credible but unconfirmed al-Qaida threat to use a car bomb on bridges or tunnels in New York or Washington. Police Commissioner Raymond Kelly said Thursday that police are beefing up security at bridges and tunnels, setting up vehicle checkpoints and doing bomb sweeps of parking garages.

New York City commuters were told to expect a show of force at Grand Central Terminal, Penn Station and the Times Square subway station because of a previously planned counterterror drill.

On Friday, Bloomberg said New Yorkers should “go back to work. And leave it to the professionals.”