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The explosives used in the Boston Marathon bombings were homemade devices full of nails and metal fragments designed to cause widespread injury, according to initial reports.
A day after an attack that left three dead and more than 170 wounded, the FBI and Boston police declined to reveal details of their probe, or whether they suspected the assault was linked to foreign or domestic extremists.
A team of about 30 bomb specialists and sniffer dogs combed the site, the finish line for the famous marathon in the northeastern US city, as officials appealed for witnesses to call in with any relevant information.
"The scene is going to take several days to process," said Gene Marquez, acting special agent in charge of the Bureau of Alcohol, Tobacco, Firearms and Explosives office in Boston.
Police said the area had been swept twice before the race and no explosives were found.
Emergency workers and doctors who treated the victims said the bombs appeared to have sent nails, ball bearings and metal fragments flying, causing injuries mostly to the lower part of the body, and forcing several amputations.
"There are a variety of sharp objects that we found in their bodies," said George Velmahos, chief of trauma surgery at Massachusetts General Hospital.
"Probably these bombs had multiple metallic fragments in them and we removed pellets and nails," he said.
The devices were "placed probably low on the ground, and therefore lower extremity injuries are to be expected," Velmahos added.
The relatively limited scale of the two explosions and the white smoke that rose up afterward seemed to rule out military-type plastic explosives such as C4 or Semtex, which tend to produce more powerful detonations with black smoke, experts said.
CNN reported the two bombs were left in a backpack and may have used "pressure cookers" and probably employed timers.
Media reports quoting unnamed investigators increasingly focused on improvised devices, possibly a pipe bomb or an acetone peroxide explosive, which are both relatively easy to make and to conceal.
An acetone or TATP explosive was employed in the 2007 London bombings, an attack on the 1996 Olympics in Atlanta and a failed attempt by "shoe bomber" Richard Reid in December 2001 to detonate a device on a Miami-bound flight.
The organic peroxide, however, is unstable and sensitive to any heat and friction, with bomb-makers often suffering injuries when preparing the devices.
Bomb-making manuals appearing in jihadist online forums, including one produced by Al-Qaeda's branch in Yemen, often suggest using pressure cookers, according to SITE Intelligence Group, which monitors extremist groups' online messages.
In 2010, the first issue of an English-language magazine by Al-Qaeda in the Arabian Peninsula (AQAP), the terror group's Yemeni affiliate, included an article explaining how to make an explosive with a pressure cooker and shrapnel.
The article was titled "Make a Bomb in the Kitchen of Your Mom" and had a photo of a backpack concealing the explosive.
White supremacists in America took note of the article and one online forum, Stormfront, called the how-to manual "highly recommended reading," according to SITE.
In 2003, the Department of Homeland Security issued a bulletin warning border agents and police to be on the lookout for extremists converting pressure cookers into bombs.
The bulletin said it was a "technique commonly taught in Afghan terrorist training camps" and that pressure-cooker bombs were used in attempted attacks or plots in India and Algeria, as well as an assault in Nepal.
"Typically, these bombs are made by placing TNT or other explosives in a pressure cooker and attaching a blasting cap at the top of the pressure cooker," the bulletin said.