The London Attack Was a Grim Reminder That Homemade Bombs Are an Ongoing Threat

London was struck Friday by its fourth terrorist incident this year, when an improvised explosive device, or IED, partially detonated on a morning rush hour train, triggering a panicked rush from the scene and leaving 22 injured but none dead.

Images of what appeared to be a homemade bomb soon began circulating on social media, showing what looked like a smoldering white bucket with wires attached. “Clearly, this was a device that was intended to cause significant harm,” Prime Minister Theresa May said following the attack.

Friday’s assault marks the latest in a disturbing trend of IED attacks throughout Europe. The Paris attack in November of 2015, the attack in Brussels last year, and the Manchester bombing this summer all involved crude homemade bombs — as did 2005’s London bombings, which killed 56 people and remain the U.K.’s deadliest-ever terrorist attacks.

IEDs have long been a weapon of choice for guerrilla fighters, insurgent groups and separatists. (Indeed, explosive devices were used throughout the Northern Ireland conflict.) But they are increasingly being deployed by anti-Western terrorists with the backing of, or inspired by, groups like ISIS and al-Qaeda.

IEDs are popular with such militants in part because construction manuals are relatively easy to find online, offered by foreign terrorist groups and far-right organizations alike. Meanwhile, the tools required to build them are typically cheap and easily sourced. And bomb attacks’ apparent randomness can inspire fear that lasts long after the smoke of any single explosion clears.

Still, experts say that even with the right manuals and equipment at hand, building an IED requires a certain level of skill gained through practice. Otso Iho, senior analyst at Jane’s Terrorism and Insurgency Centre, says that terrorists in countries like Iraq and Syria can often practice their deadly craft with impunity, which assailants in Western countries typically cannot.

“It’s difficult to build a reliable explosive device if you don’t have the required training and you haven’t actually practiced in a setting where people are building these things on a daily basis,” he says.

It’s possible that the assailant, or assailants, responsible for Friday’s attack lacked training. It’s also possible that fortune alone kept the explosion from claiming a body count. Regardless, the incident shows that London, the U.K. and Europe remain under threat of further bomb attacks.