On Monday, House Homeland Security Committee chairman Michael McCaul (R., Texas) conjectured that the two bombs used in the Boston Marathon attack may have been “pressure-cooker devices.” On Tuesday, a law-enforcement source confirmed to the New York Times that “at least one and probably both of the bombs were pressure cookers filled with nails, ball bearings, and black powder.”
Why pressure cookers? The technology, which cooks food quicker and more efficiently than conventional pots, is more than 300 years old. The idea is simple enough. When you cook food in liquid, you’re limited by the boiling point of water — 212 degrees Fahrenheit at sea level. But cook the same food in a sealed, airtight container, and suddenly the energy in the water that is usually released as steam can’t dissipate. Instead the contents are “superheated” to temperatures as high as 250 degrees at pressures of 15 pounds per square inch, permeating your stew and cutting its cooking time in half.
Of course, pressure cookers are also one of the more dangerous implements in the home kitchen, and design flaws or user error can lead to explosions that result in property damage, personal injury, and even death. So it’s not that surprising that terrorists around the world who trade in property damage, personal injury, and death would have recognized the pressure cooker’s potential in an improvised explosive device (IED). After all, fragmentation devices from hand grenades to howitzer shells operate on a similar principle. Seal an accelerant in a hard outer shell so that, instead of dissipating into thin air, the entirety of the kinetic energy of the explosive is directed through the solid shell, generating shrapnel and upping body counts. It’s the difference between setting off an M-80 in your open palm — which will certainly hurt — and setting it off in your closed fist — which will hurt much, much more.
Pressure-cooker IEDs have been used in terror attacks and asymmetric warfare for at least the last decade. A 2000 report on IEDs from a South Asian think tank noted that “fire extinguishers, pressure cookers and gas cylinders are favoured” by insurgents and terrorists “because of the additional splinter effect they can cause due to their thick metal sheeting.” This became a going concern for coalition forces in Iraq and Afghanistan early in those conflicts. In 2004, the Department of Homeland Security issued a bulletin (which Michael Crowley pointed out) to emergency personnel about terrorists converting pressure cookers into IEDs.
“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,” the bulletin notes, adding that the technique is commonly taught at Afghan terrorist training camps. Instructions for building pressure-cooker devices eventually made their way into Inspire, al-Qaeda’s magazine, and into the heads of would-be terrorists such as Faisal Shahzad, who used a pressure cooker packed with 100-plus firecrackers as part of the grab-bag bomb used in his failed attack on Times Square in 2010.
But the fact that the Boston devices reflect known tactics of Islamist operators might not tell us as much as you’d think. Pressure-cooker devices are popular precisely because they are relatively easy to assemble from widely available components. For this reason, they have made their way into the “anarchist cookbook”-style discourses that occupy the fringes of the Internet, potentially implicating any of a whole range of domestic ne’er-do-wells and sociopaths — Left, Right, and Neither — as much as they do the al-Qaedas of the world.