How pressure-cooker bombs boost the deadliness of 'low explosives'

Pressure cookers are used in a kitchen to speed up the preparation of food -- and in the hands of a terrorist they can have a similar effect, accelerating the power and lethality of a so-called low explosive.

Investigators have said they believe two pressure cookers were used in the twin blasts detonated at the Boston Marathon on Monday. The explosions, which were set off near the finish line, killed three people and injured more than 170.

Kevin Miles, the director of military contractor Troy Asymmetric and a former FBI special agent with more than 30 years of experience as a bomb technician, told CTV's Canada AM pressure cookers are used by bomb makers in places like Afghanistan, Pakistan, India and Nepal.

Similar to a pipe bomb, pressure cookers allow a bomb maker to turn a low explosive, such as black powder or smokeless powder, into a much more dangerous and deadly substance. The pot is able to contain the energy from the explosive, allowing it to build before it releases and turns the pressure cooker itself into shrapnel.

"You have to realize the explosive used in this incident most likely -- I don't know yet and no one knows for sure -- was what's called a low explosive. And in order for a low explosive to function as an explosive, it has to be confined … it has to be contained in something," Miles said. "So the pressure cooker itself was merely a container for the explosive in these IEDs."

The pressure cooker would most likely have been loaded with the explosives and a detonator, then packed with projectiles. It's believed one of the bombs was loaded with ball bearings or BBs, and the other with nails. Doctors have reported having to remove nails from the wounded, including children.

Miles investigated a blast scene in Afghanistan where a pressure cooker was used, and said they are common in that region. However, pressure cookers have also been used in bombings and bombing attempts in the U.S., including New York's Times Square in 2010 and Grand Central Terminal in 1976.

"There's an urgency on the part of some to make that leap that this might be the work of an international terrorist or an international group of terrorists," Miles said. "It's true pressure cookers have been used in others parts of the world, particularly Afghanistan, but you also have to realize pressure cookers are readily available in Canada and the U.S. in any housewares store."

Investigators haven’t yet said whether they believe the attack was the work of an individual or a group of people. No group has yet claimed responsibility.

Pieces of cookware were found at both blast sites. The bombs were reportedly hidden in black duffel bags or backpacks left at the scene.

Forensic experts are working to collect the fragments and reconstruct the bombs, in order to obtain any potential clues about the bomb maker's so-called signature.

"We basically try to create a model for what the bomb looked like," Matthew Horace, a former special agent for the Bureau of Alcohol Tobacco, Firearms and Explosives, told The Associated Press. "Investigating bombs is like a puzzle."

Denny Kline, a former FBI explosives expert and instructor in forensics at its academy, said experts are combing through the wreckage searching for the components of the bomb in order to create that model.

"It's going to change its appearance and its form, but it's going to remain," said Kline. "It'll be broken up into lots of little pieces, but it's not going to evaporate."

Investigators hope to eventually determine the make of the pressure cookers, which could provide important clues about how the attack was planned.

Other components of the bomb, such as the timer used -- in this case, experts say a simple egg timer would likely have worked -- could also help point police toward the right direction in the ongoing investigation.