Q&A: Pressure cooker bombs
Early investigations into the deadly attack on the Boston Marathon suggest that the two bombs were placed in pressure cookers.
The vessel has long been favoured by bomb-makers in Afghanistan because it can be rigged into a cheap but murderously effective anti-personnel device.
What are the advantages of a pressure cooker for a bomber?
Basically a tightly sealed kitchen pot, the pressure cooker by its very nature increases the force of the blast, and can be packed with shrapnel such as ball bearings and nails. It is also less conspicuous than pipe bombs in regions where it is commonly used in the kitchen, such as South Asia.
How common is its use as a bomb?
According to US government guidance for security officials in 2004: "A technique commonly taught in Afghan terrorist training camps is the use/conversion of pressure cookers into IEDs [improvised explosive devices]". An update in 2010 records that pressure cooker bombs have been frequently used in Afghanistan, India, Nepal and Pakistan.
Pressure cooker bombs
Who do the bombers target with such devices?
Taliban bombers in Afghanistan have placed pressure cooker bombs along paths expected to be used by US troops on foot patrol, the US defence news website Defensetech says. One of the deadliest attacks outside Afghanistan occurred in Mumbai in 2006, when seven pressure cooker bombs tore through trains in the Indian city, killing at least 186 people and injuring 700. Timers had been attached to the bombs, which were put into bags and concealed with newspapers and umbrellas.
Were pressure cooker bombs used in the US before Boston?
A pressure cooker was used in one of the bombs found after the failed Times Square bombing in New York in May 2010, according to the US government guidance from the same year. It added that because the cookers were less common in the US, their presence "in an unusual location such as a building lobby or busy street corner should be treated as suspicious". A much earlier attack featuring a pressure cooker bomb in New York was in 1976, when Croat nationalist militants planted one in a locker at Grand Central Station. It exploded as an attempt was made to defuse it and a policeman was killed.
What do we know about the Boston bombs specifically?
Investigators believe they consisted of explosives put in ordinary 1.6-gallon (6-litre) pressure cookers, one with shards of metal and ball bearings, the other with nails, a source close to the investigation told the Associated Press news agency on condition of anonymity. Stuffed into black duffel bags, the bombs were then left on the ground. They exploded within about 10 seconds of each other.
Are pressure cooker bombs associated with particular militant groups?
Gordon Corera, the BBC's security correspondent, is sceptical. This type of bomb is relatively easy to construct without much bomb-making skill or experience, which makes it possible for a lone wolf actor to have built it, he says. With no need to get hold of high explosives, it also makes it easier to do so without being spotted. Publications linked to al-Qaeda have put out "do it yourself" instructions on how to make such bombs, but that does not necessarily mean it has to be someone associated with that ideology who was behind the Boston attacks.