"The people doing these investigations are extremely knowledgeable and they have extremely sensitive instrumentation in the field and in the laboratory," said Sigman, who is also associate professor of chemistry at the university. "If they didn't already know this morning what explosives were actually used, I'd be surprised. They probably knew yesterday."
Law enforcement officials investigating the two explosions think at least one of the devices used a metal pressure cooker and was hidden in a backpack or duffel bag, according to two law enforcement officials briefed on the matter who asked for anonymity to discuss details.
The improvised bombs contained shards of metal, nails, and ball bearings to increase the carnage among race participants and spectators.
Some victims had 40 or more fragments of pellet and nail-like shrapnel embedded in their bodies, said Dr. George Velmahos, chief of trauma surgery at Massachusetts General Hospital. The fragments were uniform, indicating that they came from the two blasts and not from the surrounding environment, he said in a news briefing Tuesday.
The use of a metal pressure cooker — a variant on the more common pipe bomb — increases the lethality of the blast due to the metal shards. That type of bomb has joined the list of improvised explosive devices in recent years.
The Department of Homeland Security issued a 2004 warning about the "potential terrorist use of pressure cookers," saying the technique was "commonly taught in Afghan terrorist training camps" and also had been used in India.
"Typically, these bombs are made by placing TNT or other explosives in a pressure cooker and attaching a blasting cap at the top of the pressure cooker," says the report, which is available online. "The size of the blast depends on the size of the pressure cooker and the amount of explosive placed inside. Pressure cooker bombs are made with readily available materials and can be as simple or as complex as the builder decides. These types of devices can be initiated using simple electronic components including, but not limited to, digital watches, garage door openers, cell phones or pagers."