The Associated Press has reported that the explosives used to bomb the Boston Marathon on Monday were homemade pressure cooker devices.
A person who spoke on the condition of anonymity because the investigation was still going on said that the explosives were put in 6-liter pressure cookers, placed in black duffel bags and left on the ground. They were packed with shrapnel, the person said.
The devices are apparently very easy to make, as one simply loads explosives into the pressure cooker and adds a blasting cap to the top. The Department of Homeland Security issued a warning about terrorist pressure cooker bombs back in 2004.