At the event on Monday, 3 people, including an 8-year-boy, in addition to more than 150 others were injured by two explosions just seconds apart near the finish line.
Special Agent in Charge Richard DesLauriers says at a news conference there are no known additional threats.
Police commissioner Ed Davis says it is the most complex crime scene in history of the department.
Authorities are looking for amateur video and photographic evidence that can give clues to who set off the bombs.
Suffolk County District Attorney Daniel Conley says “what occurred in Boston was an act of cowardice.