The probe of the Boston Marathon bombings has led investigators to suspect that pressure cookers were used in the attack that killed three people and maimed dozens others.

The Federal Bureau of Investigation's special agent in Boston Richard DesLauriers said items recovered at the scene included pieces of black nylon that could be from a backpack, fragments of ball bearings and nails, and the possible remains of a pressure cooker device.

DesLauriers also said the FBI is currently hearing testimony from witnesses and studying the scene of the attack. The explosions went off near the finish line of the race and sent more than 170 people to the hospital.