MSNBC
Gov Romney, Mass.: 5 US Cities have been asked to raise their security on their transit systems.
Major U.S. cities raise vigilance
No immediate plans to raise threat level
Department spokesman Brian Roehrkasse said, however, that there were no immediate plans to raise the nation's threat level.
"We do not have any intelligence indicating this type of attack is planned in the United States," Roehrkasse said.
The Homeland Security Department asked authorities in major cities for "continued vigilance" of their transportation systems, Roehrkasse said.