Authorities investigating the Boston Marathon bombings say they have recovered a number of pieces of evidence from the scene.

The FBI says the bombs used to kill three people and injure over 170 at yesterday's race contained 'BB pellets and nails' - possibly stored within a pressure cooker.

No one has claimed responsibility for the attack - and the agency says the list of possible suspects is wide open.

FBI Special Agent Richard DesLauriers says they're looking to speak to people who may have seen anyone carrying a heavy, black nylon backpack near the scene.