April 16, 2013

Pressure cookers make strong bombs, good clues

The pots that speed the cooking of beans and meat also make weaker, easy-to-obtain explosives much stronger.

The Associated Press

WASHINGTON — In kitchens, they prepare food faster, but pressure cookers by their very nature help make good bombs, amplifying the blast and the carnage.

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Investigators in haz-mat suits examine the scene of the second bombing on Boylston Street in Boston Tuesday, April 16, 2013 near the finish line of the 2013 Boston Marathon, a day after two blasts killed three and injured over 170 people. (AP Photo/Elise Amendola)

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They don't just hold the explosives. The tightly sealed pot that speeds the cooking of beans and meat makes easier-to-obtain weaker explosives faster and stronger. And they may also help investigators find out who built the deadly homemade bombs that exploded at the Boston Marathon on Monday.

Investigators found fragments of BBs and nails, possibly contained in a pressure cooker, said Richard DesLauriers, the FBI agent in charge in Boston. He said the items were sent for analysis.

If a pressure cooker was used, it probably cost around $100 to construct, say former federal forensic and explosive investigators. It's like a pipe bomb but bigger and more powerful.

Pressure cooker bombs are more often used in Afghanistan, Pakistan India, and Nepal — where the pots are more commonly used for cooking. But they have also been prominent in bombings and attempts in the United States, especially in New York in Times Square in 2010 and Grand Central Terminal in 1976.

In Al Qaeda's online magazine, there's even an article titled: "Make a bomb in the kitchen of your mom" by "The AQ Chef." It mentions, even recommends, pressure cookers, noting that weak explosives only work with the high pressure of a cooker or sealed pipe.

Low power explosives like black power and smokeless powder — the most likely ones used in Boston — blow up at a slower rate and only deliver the big boom if they are confined and the pressure from the gas and explosion builds up, said Denny Kline, a former FBI explosives expert and instructor in forensics at its academy.

Kline and other ex-government experts who have no role in the investigation differ about what type of explosive may have been used and some refuse to even speculate what kind.

The pressure cookers are a key first piece in a painstaking detective process. The sound of the explosion is a clue. The color of the flash — yellow — and smoke — white — are clues. So is the size of any crater and the distance fragments flew. Even the smell can give a seasoned investigator a good idea of what explosive was used, Kline said.

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

Piece by piece, forensic investigators now have to put together what came apart with an explosive force of thousands of feet per second: The bombs themselves.

"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."

The job is to piece things back together and identify chemicals. But it happens slower than on TV crime shows. And it isn't as easy, Kline said.

"It takes a lot more intelligence to put it back together... from multiple pieces than to follow a simple set of instructions on the Internet," said Roy Parker, a retired ATF explosives expert.

Kline said once forensic investigators have something on the bomb itself, it is given to lead detectives to take the next big step

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