The bombs that exploded at the Boston Marathon were likely heavy, carried to the scene in dark nylon bags, and fashioned out of pressure cookers packed with shrapnel to make them more lethal, authorities said on Tuesday (April 16).

Bomb scene pictures taken by investigators and released on Tuesday show the remains of an explosive device including twisted pieces of a metal container, wires, a battery and what appears to be a small circuit board.

One picture shows a few inches of charred wire attached to a small box, and another depicts a half-inch (1.3 cm) nail and a zipper head stained with blood. Another shows a Tenergy-brand battery attached to black and red wires through a broken plastic cap. Several photos show a twisted metal lid with bolts.

A U.S. government official, who declined to be identified, made the pictures available to Reuters.

At the scene of the marathon, investigators recovered material that was being reconstructed at the FBI laboratory in Qua