As FBI and law enforcement officials continue to search for a suspect and motive in Monday's attack on the Boston Marathon, new details are emerging about the source of the explosions.
According to multiple reports, investigators now believe the explosive devices were shrapnel-packed pressure cookers. The Associated Press, citing a person close to the investigation, said that the 6-liter cooking utensils were loaded with metal shards, ball bearings, and nails, and then stashed in duffel bags planted along the race route.
Fox News, also citing officials close to the investigation, said shredded pieces of pressure cookers were found at the blast sites. Those sources also claimed that investigators believe the pressure cookers themselves could have served as detonation timers, or that they may have been activated remotely via cell phone. The FBI is reportedly searching cell phone records to see if they can identify a remote detonation signal.
Yet another law enforcement official, speaking to CNN, insists that a timer was likely used to detonate the bombs, not a cell phone.
Leaving aside how the devices were exploded, early reports from the scene Monday indicated that victims appeared to have been riddled with small pieces of metal, consistent with the damage typically associated with shrapnel-filled explosive devices. On Tuesday, confirming those early reports, George Velnahos, chief of trauma surgery at Massachusetts General Hospital, said doctors there had found BBs, pellets, shrapnel, and nails inside victims' bodies, adding that some people had as many as 40 or more pieces of metal lodged inside them.
The pressure cookers presumably responsible for that carnage are actually a quite common medium for improvised explosive devices. Such homemade weapons have been used in attacks in India, Algeria, Nepal, Pakistan, and Afghanistan, according to the Department of Homeland Security. Seven pressure cookers were used in the 2006 Mumbai train attacks that killed more than 200 people. And a pressure cooker was also one of several explosives involved in the attempted bombing of Times Square in 2010.
In fact, the devices are so ubiquitous as instruments of terror that the Department of Homeland Security has been on the lookout for them since at least 2004. A DHS memo from that year explaining how the devices work reads, in part:
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. As a common cooking utensil, the pressure cooker is often overlooked when searching vehicles, residences or merchandise crossing the U.S. Borders. [Department of Homeland Security]
The agency, in conjunction with the FBI, again warned of the device's damaging potential in a 2010 memo. "Placed carefully, such devices provide little or no indication of an impending attack," the memo reads.
After Monday's attack, the top Republican on the House Homeland Security Committee, Rep. Michael McCaul (Texas), told The New York Times that investigators he had spoken to confirmed the marathon explosives were similar to IEDs used on troops in Iraq and Afghanistan. But investigators reiterated Tuesday that they still have no evidence to support the notion that the Boston bombing was a foreign-led terror attack. As TIME's Michael Crowley notes, pressure cooker bombs are incredibly easy to make, so it's too early to speculate about who could have constructed these explosives.
"Counterterror officials are surely well aware of these facts and studying any leads that might link the device in Boston to Islamic radicals here or abroad," says Crowley. "But it's important to bear in mind that the ability to make these bombs is hardly unique to al Qaeda and its sympathizers. Details on how to make a pressure cooker bomb can also be found on websites associated with anarchy and other forms of non-religious radicalism."