Investigators Are Looking At These Items To Find The Boston Marathon Bomber

Investigators have several pieces of evidence believed to be part of the homemade explosive devices used in Monday's bombing at the Boston Marathon.

By examining the physical remnants found at the scene and poring through a mountain of photos and video of the event, authorities will to attempt to identify who placed the bombs near the finish line.

The attack — involving two blasts 12 seconds apart — killed three people and injured more than 180 others, many of whom now live with severed limbs.

Photos of the crime scene show the remains of a pressure cooker, a shredded black backpack, pieces of shrapnel, a battery, and a partial circuit board.

The FBI, which released the photos, said one bomb involved a pressure cooker — packed with metal including carpenter nails and ball bearings — hidden inside a dark-colored nylon bag or backpack.

U.S. Government via Reuters A nail, a zipper pull, and ball bearings used as shrapnel in a bomb

On Wednesday morning CNN reported, citing a law enforcement source, that investigators have found the lid to a pressure cooker on one of the rooftops in the area — which could hold a number of clues for technicians.

The agency said that a metal container hidden in a backpack also housed the second bomb, but there's not enough evidence to determine whether it was also a pressure cooker.

A law enforcement official told CNN that the bombs were likely detonated by timers connected to the circuits and batteries, but the FBI said they haven't figured out the detonating system or what exactly set them off.

U.S. government via Reuters Circuitry found at the scene

Technicians at FBI's national laboratory in Virginia will now try to reconstruct the devices using fragments of the pressure cooker, nails, and nylon bags in addition to the type of circuitry and batteries found at the scene.

Several reports detail al-Qaeda's proliferation, through "Inspire" magazine, of how to build pressure cooker bombs. But it's important to note that such knowledge was published in the Anarchist Cookbook in 1971 and is widely available on the Internet.