The US raised its terrorism alert level to high or code orange for the entire transport system after suspected Islamic terrorists triggered a series of blasts in London killing 37 people and injuring 700.
The Department of Homeland Security raised the alert level to the second highest category of five for railway and subway systems, as well as intra-city bus lines. Airline traffic remained unaffected.
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"We feel that, at least in the short term, we should raise the level here because, obviously we are concerned about the possibility of a copycat attack," Homeland Secretary Michael Chertoff told reporters.
But he assured the nation that US authorities have 'no specific credible evidence' pointing towards an attack on the country. At the same time, he said, "we are also asking for increased vigilance, particularly in the US transportation system."
He stressed that the government is not asking Americans to avoid using their subways and bus systems. To the contrary, he said, those who use mass transit should continue to do so.
Raising the alert level to orange means that local officials are urged to implement heightened security measures such as increasing police patrols, inspecting cars and using bomb-sniffing dogs.
At least two Americans were among the 700 people injured in the London attacks but there were no reports of US
fatalities, a State Department official said.