An unnamed FBI official has told CBS News that at least one of the explosive devices detonated in Boston yesterday appears to have been improvised from a conventional pressure-cooker. Unnamed law enforcement officials don't exactly have the strongest record of credibility in the immediate aftermath of events like these, but federal authorities are well-acquainted with this type of IED.

In mid-2010, the Department of Homeland Security put out a circular, classified "For Official Use Only," warning of their potential use in domestic attacks:

Rudimentary improvised explosive devices (IEDs) using pressure cookers to contain the initiator, switch, and explosive charge (typically ammonium nitrate or RDX) frequently have been used in Afghanistan, India, Nepal, and Pakistan. Pressure cookers are common in these countries, and their presence probably would not seem out of place or suspicious to passersby or authorities. Because they are less common in the United States, the presence of a pressure cooker in an unusual location such as a building lobby or busy street corner should be treated as suspicious.

The notice (full text below) adds that the failed Times Square bomber used a similar device. And it serves as an update to a 2004 DHS bulletin, also embedded below, citing the existence of such devices in Afghanistan, Pakistan, Nepal, and Malaysia.

Why a pressure cooker? As the DHS alert suggests, it's a relatively nondescript item, not likely to arouse immediate suspicion to the untrained eye. It can conceal a good deal more explosive and a larger initiating mechanism than, say, a pipe bomb—whose appearance is obvious enough to draw greater scrutiny. Pipe bombs also can't hold much in the way of shrapnel—nails, BBs, ball bearings—but a pressure cooker makes it easier to carry and conceal such lethal add-ons.

Indeed, these explosives are common enough that some explosive ordnance disposal trainers sell mockup devices to law enforcement. They are familiar to US troops in Afghanistan; the two videos (above and below) are good examples of what a controlled detonation of a pressure-cooker explosive reportedly looks like.

This does not, however, confirm that the Boston bombs have any specific provenance tied to South Asian terrorist or insurgent groups. This might not have come from a group at all:

  • The lack of clear leads on Day Two suggests that law enforcement has not isolated any chatter from these organized syndicates—even chatter that might not have been specific enough to predict the attack, but whose meaning might become clear in the explosions' aftermath.
  • There were only two bonafide bombs, notwithstanding all the "device" rumors that swirled around Boston and the media yesterday, so while it was a terrible planned crime of opportunity, it ranks low on the spectrum of complex coordinated attacks.
  • The only individual whose physical appearance might be said to resemble that of a South Asian—in actuality, he's reportedly Saudi—was tackled, interrogated, and thus far not found to be connected to the attacks.
  • Downrange American military personnel and the civilian contractors they work with also return home with the technical know-how to create these devices. Sinister knowledge isn't proprietary to an ethnic or religious group.

More on that last point: Improvised shrapnel munitions, whether they are staged in a pressure cooker or not, are easy to build with online directions or books for sale at gun shows. I know—as a teen, I bought an old copy of the Army's "improvised munitions" manual, repackaged as a "CIA Black Book of Dirty Tricks," when I accompanied my father to a gun show more than two decades ago.

These points—and the fact that the attacker's target appears to have chosen for mass casualties, rather than for symbolic political value—suggest (but do not prove) that, whether the perpetrator is a white nationalist or an Islamic jihadi or something else entirely, he or she could be unaffiliated and off the grid.

But deeper speculation on an attacker's ethnicity, creed, or possible affiliation is just that. At this premature stage, we can only say one thing authoritatively about the Boston bombers: They are sick bastards with a basic understanding of physics.

The 2012 DHS alert:




The 2004 warning: