Michael Crowley delivers "A short recent history of pressure cooker bombs" in Time Magazine:

As it happens, pressure cookers have a nefarious reputation in counterterrorism circles. In 2004, the Department of Homeland Security was concerned enough about pressure cooker bombs to issue an alert to federal and state security officials: “A technique commonly taught in Afghan terrorist training camps is the use/conversion of pressure cookers into IEDs,” the bulletin warned.

That bulletin cited several plots from 2002 to 2004 to use pressure cooker bombs in France, India and Nepal. But more recently there have been at least three other instances of would-be terrorists in the west, all of them Islamic radicals, in possession of pressure cookers for reasons that seemed not to involve having friends over for dinner ...

But it’s important to bear in mind that the ability to make these bombs is hardly unique to al Qaeda and its sympathizers. Members of at least one prominent white supremacist website have shared terror tips from Inspire [a radical Islamic magazine] which one called “highly recommended reading.” Pressure cooker bombs are also discussed in detail on this anarchist site, which describes how to build what is “affectionately known as a HELLHOUND.”