Three people were killed, including an 8-year-old boy, and 176 were injured by two explosions just seconds apart near the finish line.
Special Agent in Charge Richard DesLauriers says there are no known additional threats and agents are following a number of leads.
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.
