Indeed, there's a long history of terrorists employing pressure cookers abroad. On Tuesday morning Time's Michael Crowley surfaced a 2004 memo issued by the Department of Homeland Security, cataloging episodes in which Afghan terrorists used pressure cookers to fabricate explosive devices and detonated them in several countries throughout the 2000s. (Such contraptions are especially prevalent in contested regions of Pakistan.)
American soil is no stranger to these contraptions, either. The foiled terrorist plot to detonate a bomb in Times Square on May 1, 2010, involved a pressure cooker.
Indeed, nobody in law enforcement or government is making any public connections to any groups or places right now. A Saudi man reported to be a "person of interest" has been cleared as a suspect. As President Obama said at the White House Tuesday: "What we don't yet know, however, is who carried out this attack, or why; whether it was planned and executed by a terrorist organization, foreign or domestic, or was the act of a malevolent individual. That's what we don't yet know. And clearly, we're at the beginning of our investigation."
In a post expanding on the memo, Time's Crowley noted the ease of creating such an explosive device. "The ability to make these bombs is hardly unique to al Qaeda and its sympathizers. Details on how to make a pressure cooker bomb can also be found on websites associated with anarchy and other forms of non-religious radicalism," he said, before linking to such a website.
Other evidence suggests that the bombs were made with readily-available chemicals. New Scientist asked a bomb expert to study the footage of the explosion for clues of the bombs origin. Here's what the expert concluded: "The Boston bombs ... produced an orange flash followed by white smoke. This suggests the bombs contained neither TNT nor RDX, which produce black smoke, but could have been a home-made chlorate-based device."
Update, 5:04 p.m.: ABC News is reporting that one of the bombs used gunpowder to detonate.
It's unclear how the bombs detonated
In the same report detailing how the bombs were built, the Associated Press notes, "law enforcement officials have some of the bomb components but [do] not yet know what was used to set off the explosives." In that vacuum of knowledge a number of rumors have appeared: 1) the bombs used a timer; 2) the bombs were activated remotely; 3) cell phone service was shut down in downtown Boston to avert other remotely-activated bombs; and 4) a mysterious roof-top figure captured in a single photo has something to do with setting off the bomb.
Rumor No. 1 appears to be the strongest. After speaking to an official close to the investigation, CNN knocked down rumors (discussed below) that the bomb had been detonated using a cell phone-activated trigger, and reported that investigators in Boston think the bombs had timers attached to them instead. And, within minutes of CNN's report, the Globe reported that investigators had turned up a circuit board they believe was used to trigger the explosions; it was reportedly found near a spot where Gov. Patrick had been seated earlier in the day.