The bombs used to attack the Boston Marathon appear to have been homemade improvised explosive devices (IEDs) made with components easily available in chain big box retail or hardware stores. The Department of Homeland Security warned in 2010 about the devices finding their way to the U.S. after seeing them used in insurgencies in Afghanistan, India, Nepal, and Pakistan.
Surgeons at hospitals in Boston who worked on bomb victims told the Boston Globe that they removed large numbers of pellets, small BBs, and headless nails from patients. These bombs, say a federal law enforcement official, were made out of six-liter pressure cookers stuffed with a "low-velocity improvised explosive mixture," shrapnel, and a timer.
"IEDs represent the most ubiquitous threat to our nation for those intent on conducting physical attacks. Law enforcement and the private sector need tools to provide the capability to prevent such incidents," Robert Liscouski of Implant Sciences told Fast Company. Implant Sciences is a Boston-area company that manufactures portable explosives detectors.
An unclassified Department of Homeland Security infosheet from 2010 sent to law enforcement officials nationwide warned of pressure cooker bombs. Federal officials expressed concern that homegrown American terrorists or overseas fighters could bring use of the weapons home. Crude IEDs with initiators, shrapnel, and explosives hidden inside can be constructed with relatively little effort; a quick Google search found multiple websites and torrents offering instructions on how to construct them. The aborted 2010 Times Square bombing also included a crude IED which used firecrackers stuffed into a pressure cooker.
The best known use of a pressure cooker bomb was in 2010, when terrorists suspected of Taliban links blew up the offices of World Vision, a Christian charity operating in Pakistan. In a pressure cooker IED-using explosion in the northwestern village of Oghi, terrorists used the remote-detonated bomb to kill six. "They left a locally made pressure-cooker bomb that exploded soon after the attackers fled the scene, killing NGO people first by gunfire and then with the blast," World Vision's Liaquat Shah told the BBC.
In the law enforcement warning, the Homeland Security Department said that the innocuous appearance of pressure cooker bombs helped their deployment overseas. Instead of registering the bomb as a potential threat, bystanders and law enforcement were tricked into thinking of them as a harmless piece of trash. Potential warnings of a pressure cooker bomb, the agency says, include "wetness or unusual stains" on a container or abandonment of kitchen equipment in high-traffic locations.
Homeland Security specifically warned local law enforcement to keep an eye out for pressure cookers in building lobbies or in street corners and said that any placement there was "suspicious." Both bombs used in Boston appear to have been pressure cookers hidden in public areas alongside the marathon route.
The documents indicating worry about pressure cooker bombs were first posted in 2010 to Public Intelligence, a website dedicated to making government information public.
Homeland Security, however, warned employees about pressure cooker bombs even before 2010. The Washington Post's Caitlin Dewey reports an internal memo described pressure cooker bombs in 2004 and described their construction in detail.
[Pressure Cooker Image: Flickr user sam_churchill]