A total of 176 victims injured in Monday’s bomb attack in Boston have been placed in hospitals around the city, with 17 of them in critical condition, according to a police official.

Boston Police Commissioner Ed Davis has described the crime scene as “the most complex in the history of the department.”

Davis told a press conference there were two sweeps for explosives before the bombing, one in the early morning and one just before the lead runners came through.

Those sweeps turned up nothing, he said, but because there is unrestricted access to the course someone could have planted a device later.

Three people were killed in the two bomb explosions at the finish line of the Boston Marathon on Monday. The explosions occurred at about 2:45 p.m. Eastern Time, more than two hours after the first of the race's nearly 27 000 runners had crossed the finish line.