The six-litre pressure cookers stuffed with nails, ball bearings, other shrapnel and gunpowder were placed on the ground inside black duffel bags near the finish line.
According to investigators, the devices of such type have been used in attacks in Afghanistan, Pakistan, India and Nepal, and one was also deployed in the failed Times Square bombing three years ago.
However, as The Telegraph reports, the Pakistani Taliban, which claimed responsibility for the New York attack, denied its involvment in the Boston bombings.
Those responsible for the attack could also have worked out how to make the bombs using the internet. An article in Inspire, al-Qaeda’s English-language online magazine, had earlier released an instruction which explains how to “Make a bomb in the kitchen of your Mom”, which employed a pressure cooker.
Bill Daly, a former FBI investigator, said: “I don’t think the use of pressure cookers points to any particular group or individual.”
On Tuesday President Barack Obama prepared Americans for a potentially long wait to find those responsible, saying: “What we don’t yet know is who carried out this attack or why, whether it was planned and executed by a terrorist organisation, foreign or domestic, or was the act of a malevolent individual. We will find whoever harmed our citizens and we will bring them to justice.”
Richard DesLauriers, head of the Boston FBI office, said: “This will be a worldwide investigation. We will go to the ends of the earth to find the subject or subjects responsible for this despicable crime.”
He went on, revealing that the bombs “had been carried to the site inside black nylon bags and that fragments of the explosives would be sent to a crime lab in Virginia to be reconstructed.”
He also added no one had claimed responsibility and that “the range of suspects and motives remain wide-open”.
“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,” explains The Huffington Post.
Even the smell can give a seasoned investigator a good idea of what explosive was used, said Denny Kline, a former FBI explosives expert and instructor in forensics at its academy.
“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.”
“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.”
Now investigators are expected to piece things back together and identify chemicals, which can take a lot of time and efforts to do.
“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.