After a deadly series of bomb attacks in London's mass transit system today killed at least 37 people, Department of Homeland Security Secretary Michael Chertoff announced that DHS has raised the terrorism threat level code from yellow (for significant risk) to orange (high risk) for the U.S. mass transit system.

At a press conference, Chertoff emphasized the heightened alert applies only to the transportation sector and specifically to "regional and inter-city passenger rail, subways and metropolitan bus systems."

"Currently, the United States has no specific, credible information suggesting an imminent attack here in the United States," Chertoff said. "However, we know the tactics and methods of terrorists as demonstrated by the horrific rail bombings last year in Madrid. The intent of al Qaeda and its affiliated organizations to attack in Europe and in the United States has been well documented and continues to be reflected in intelligence reporting."

The International Association of Fire Chiefs sent an alert to members, asking the fire and emergency services leaders to review preparations and to meet with local law enforcement counterparts.  IAFC President Chief Bob DiPoli urged fire department officials to review IAFC recommended actions to the Homeland Security Advisory System.