Explosives experts say pressure cooker bombs are Improvised Explosive Devices. Highly combustible material gets sealed into an airtight pot.
"It's the difference between packing explosive material in a pipe bomb as opposed to wrapping it in a paper firecracker," Bob Morris said.
Morris is a retired U.S. Army Colonel who deployed to Iraq. After 31 years in the service, he founded an organization based in Yorktown, VA, called Global Campaign Against Improvised Explosive Devices. The non-profit focuses on the danger posed by IEDs.
"The U.S. Department of Homeland Security issued a memo in 2004 warning about the threat of pressure cooker bombs," he said.
When the pressure cooker explodes, casing turns to shrapnel and fragments. The size of the pressure cooker determines the size of the blast. Morris suspected a pressure cooker bomb was used in Boston by what he saw.
"That's a classic textbook IED network attack of multiple, simultaneous IEDs in order to increase the number of casualties," he said.
The bombs are easy to make. Two years ago an online magazine linked to al-Qaida posted instructions on the Internet. The Army private convicted last year of plotting to blow up a restaurant at Fort Hood had bomb making materials in his hotel room, including a pressure cooker.
"They're just one of a myriad of IEDs that are being made out there right now," Morris said. "There's over 500 IED incidents a month around the world, not counting Afghanistan and Iraq."
Pressure cooker bombs have been used in roadside attacks in Afghanistan and bombings in other parts of the world. According to law enforcement, Boston is the latest site on the list.