Investigators were also scrutinizing surveillance video and forensic evidence after a bicycle-riding bomber struck the landmark station Thursday, scarring one of the world's most recognizable locations.

Authorities said there was no connection between the blast and a letter sent to as many as 100 members of Congress bearing the words "Happy New Year, We Did It."

The lengthy anti-war letters — which arrived with photos of a man standing in front of the recruiting office before it was damaged — contained no threats, officials said.

Laura Eimiller, an FBI spokeswoman in Los Angeles, said an individual was questioned there about the letters to Congress and "there is no evidence linking the letters, which contained no threat, to the bombing."