Investigators have been conducting a painstakingly thorough search of the area where two bombs exploded on Monday at the Boston Marathon, combing the barricaded streets for clues as to who might be behind the deadly attacks and exactly how they were carried out. U.S. counter-terrorism officials have confirmed that the bombs placed Monday at the marathon were made from pressure cookers, a crude kind of explosive favored by insurgents in Pakistan and Afghanistan. See photos from the crime scene.
Members of the FBI work through the night near the scene of twin bombings, which exploded near the finish line in Boston's Copley Square.
Spencer Platt/Getty
Officials say the bombs detonated at the Boston Marathon finish line on Monday were low-grade, improvised explosive devices packed with nails and BBs into pressure cookers—a crude kind of explosive favored by insurgents in Pakistan and Afghanistan. Why use a common kitchen tool to build a bomb? First, they're easy to build and inconspicuous (the explosives were made to look like discarded material.) Second, the same principle that increases the boiling point inside a pressure cooker also can be used to amplify the force of an explosive.
Pictured here, investigators show the remains of another explosive device.
Reuters, via Landov
Another investigator searches for evidence in the detritus near the scene of the bombings.
Officials pore over nails BBs, and other lethal shrapnel on blood-smeared streets.
Two men in hazardous materials suits put numbers on the shattered glass and debris as they investigate the scene at the first bombing on Boylston Street.
