How Do Pressure Cooker Bombs Work?

We still don’t know a whole lot about the explosion that occurred Saturday night in the Chelsea neighborhood of Manhattan. According to authorities, the explosion does appear to be the result of an intentional bombing, though nobody has yet claimed responsibility for the blast, which left 29 people with mostly non-serious injuries (Update: Authorities have identified this man, wanted for questioning).

The precise nature of the bomb itself — if indeed it was a bomb, which still shouldn’t be taken as absolutely confirmed until we know a bit more — is currently unknown, but the discovery of a possible second device a few blocks up points toward a particular kind of explosive: a pressure cooker bomb.

That second device, which was removed by a robot, reportedly resembled the devices used in the 2013 Boston Marathon bombing. Again, we don’t know if the explosion was caused by a similar device, or even necessarily if the explosion and this device — if it’s actually a device at all — are related or the product of some bizarre, admittedly unlikely coincidence. And, while it’s difficult to talk about explosions in 2016 without mentioning terrorism of some sort, we don’t yet have any evidence linking the explosion to any organization, or — because this isn’t quite the same thing — to any individuals claiming allegiance to a cause or organization.

But with all those important caveats in mind, it’s worth understanding exactly what we’re dealing with when we talk about pressure cooker bombs, especially since they can be unnervingly easy to construct.

The whole point of a pressure cooker, at least when used for its intended purpose, is that placing water under pressure increases its boiling point, which in turn makes it possible to cook food at higher temperatures — and, by extension, at higher energies. If the airtight seal on a pressure cooker fails, all that extra energy gets released with a whole lot of force, which is why a pressure cooker being used for normal cooking can be dangerous enough if something goes wrong.

When, on the other hand, the pressure cooker has explosive material placed inside it — accompanied by nails, ball bearings, and other readymade shrapnel — the blast is far deadlier. A simple electronic device from an alarm clock to a cellphone can trigger the explosion, which blows the cooker open and fires the shrapnel in all directions at great speed.

The grim appeal of pressure cooker bombs is that, unlike most improvised explosive devices (IEDs), it really doesn’t take a lot of resources or know-how to build them. All the materials apart from the explosive material itself are easily obtained, and the basic physics of the pressure cooker mean even the explosive material doesn’t have to be especially powerful to create a big explosion.

Pressure cooker bombs have their origins in the Nepalese Civil War of the 1990s. They gained greater prominence in Afghan terrorist training camps in the early 2000s, which led to numerous such explosions in Afghanistan, Pakistan, and India. The deadliest attack using pressure cookers occurred on July 11, 2006, when seven explosions on trains around Mumbai, India, left 209 dead and more than 700 people injured.

Such a coordinated attack, however, isn’t necessarily reflective of the main reason terror organizations have so prized pressure cooker bombs. Instead, as the Al-Qaeda-linked magazine Inspire made clear in a 2010 article detailing how to build such explosives, these bombs were ideal weapons for individual terrorists looking to carry out isolated acts of destruction. That was the case with the Boston marathon bombing, as well as failed bombing attempts in New York’s Times Square in 2010 and at Fort Hood, Texas, in 2011.

Once more, we don’t yet know the precise nature of last night’s explosion, or whether pressure cooker bombs had anything to do with it. We’ll likely know a lot more over the coming days. But even leaving aside what happened last night, pressure cooker bombs have proven themselves a grim mainstay of modern terrorism.