Few Americans are likely to be familiar with pressure cooker bombs, the explosives reportedly used in Monday’s Boston Marathon attacks – but the Department of Homeland Security has been on alert for the devices since at least 2004, according to an internal memo from that year.

The almost decade-old memo described pressure cooker bombs as "a technique commonly taught in Afghan terrorist training camps."

To be clear, there’s no evidence linking Monday's attacks to foreign terrorists. Just as important to keep in mind is that a great deal may have changed in the nine years since DHS released the memo, particularly in terms of who uses these sorts of devices and where.

The memo goes on:

Typically, these bombs are made by placing TNT or other explosives in a pressure cooker and attaching a blasting cap at the top of the pressure cooker. The size of the blast depends on the size of the pressure cooker and the amount of explosive placed inside. Pressure cooker bombs are made with readily available materials and can be as simple or as complex as the builder decides. These types of devices can be initiated using simple electronic components including, but not limited to, digital watches, garage door openers, cell phones or pagers. As a common cooking utensil, the pressure cooker is often overlooked when searching vehicles, residences or merchandise crossing the U.S. Borders.

The 2004 memo mentions foiled plots in France, Nepal and Jammu, India. A series of seven pressure cooker bombs killed more than 200 people in India in 2006, during the Mumbai train attacks.