By Seth Borenstein
In kitchens, they prepare food faster, but pressure cookers by their very nature help make good bombs, amplifying the blast and the carnage.
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 US$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 powder 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 colour 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
Take the pressure cooker. If the brand is determined, "investigators will track every store that sells that pressure cooker and when it was built and sold", Horace said. "This kind of investigation requires hundreds, if not thousands of leads to be followed up on."
Horace and others are confident that the pressure cooker identification can be a big help.
The pressure cooker can also help point to the type of explosive, Kline said. If it's a high powered explosive like dynamite or C4, the blast would have shattered the cooker leaving sharp edges. If it's the low explosive, it will merely blast through, leaving more squared off edges, he said.
Once everything is pieced together, investigators will look for the "signature" or style of a bomber. Often - but less so since the internet was born - a signature can lead to a bomber, Kline said.
"It's like a piano player," Kline said. "You can give Dave Brubeck or Chopin the same piece of music and it will sound different."
With this type of bomb, it can be triggered with something as simple as an egg timer or alarm clock, Parker said. Experts doubt a cellphone was used.
The use of nails, shards of metals and ball bearings also amplifies the personal devastation, experts said.
"We've removed BBs and we've removed nails from kids. One of the sickest things for me was just to see nails sticking out of a little girl's body," said Dr. David Mooney, trauma chief at Boston Children's Hospital, which treated 10 blast victims.