We're still awaiting the who and the why of yesterday's Boston Marathon bombing, but it appears as though federal investigators may be getting closer to the what. Government officials who have been briefed on the investigation tell the New York Times (and presumably the Associated Press) that authorities now believe that at least one, and possibly both, of the exploisions were caused by bombs built using pressure cookers containing nails, ball bearings, and shards of metal as shrapnel. The explosives, the sources say, were likely hidden in some sort of bag left along the race course.
The Associated Press provides a quick refresher on where we've seen this type of homemade bomb before:
These types of pressure cooker explosives have been used in Afghanistan, India, Nepal and Pakistan, according to a July 2010 joint FBI and Homeland Security intelligence report. One of the three devices used in the May 2010 Times Square attempted bombing was a pressure cooker, the intelligence report said. "Placed carefully, such devices provide little or no indication of an impending attack," the report said.
According to the Department of Homeland Security, turning a pressure cooker into an IED is a technique "commonly taught in Afghan terrorist training camps," a fact that is sure to prompt additional speculation that the attack was planned by an international terrorist group. That said, we don't have to go back very far in our memory for proof that initial speculation often proves false. Only yesterday some media outlets were reporting that a Saudi national was a suspect in the case. Turns out, he's now being treated as a witness and the FBI says they have no reason to believe he was involved. So for now, we should treat the pressure cooker as one clue in a series of many, none of which have so far answered the question of who planned the attack and why.
The Pakistani Taliban, which claimed responsibility for the 2010 Times Square plot, has denied any role in the Boston Marathon attack.
Here's the DHS with more info on pressure cookers-as-bombs:
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. 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.
Read more on Slate about the Boston Marathon bombing.