The nation's 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-Qaida car bomb plot aimed at bridges or tunnels in New York or Washington. Officials say unattended cars parked near critical infrastructure will be towed.

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."