What Are ‘Pressure Cooker’ Bombs and Why Do Terrorists Use Them?

Prakash Singh/Agence France-Presse â€" Getty Images A forensic expert collecting samples from a damaged train coach after a series of bomb blasts in Mumbai, Maharashtra on July 12, 2006.

NEW DELHIâ€" “Pressure cooker” bombs,like those that officials said Tuesday were used at the Boston Marathon, are common in South Asia because they are easy to make and can maximize injuries and fatalities, terrorism experts in India said.

Pressure cookers, which are heated on stove tops and use pressured steam to speed up the cooking of items like lentils or chickpeas, were used in the Mumbai train blasts of 2006 that killed more than 100 people, a blast in Varanasi in 2006 that killed five, and they were believed to have been used in 2005 blasts in Delhi’s Sarojini Nagar neighborhood that killed dozens.

Outside India, a pressure cooker was used in a March 2010 blast in Pakistan that killed six, in the attempted bombing in New York’s Times Square in 2010 and to target Western troops in Afghanistan, among other incidents.

Paula Bronstein/Getty Images A man working with a pressure cooker in his kitchen in Kabul, Afghanistan on Oct. 1, 2002.

Terrorists favor the ordinary kitchen device because they are so ubiquitous, and because they have a tight-fitting lid, which allows bomb-makers to pack shrapnel and explosive material closely, maximizing its acceleration after an explosion, and therefore injuries, defense experts in India said. Often, they are filled with explosive material like gunpowder, metal items like ball bearings or nails and a detonator that can be remotely triggered.

“It is not a very sophisticated technology,” said J. N Roy, former additional director of India’s Intelligence Bureau, in a telephone interview. “There is not much training required to make a pressure cooker bomb.”

FBI, via Associated Press An FBI image showing the mangled remains of a pressure cooker that was used as a bomb which exploded at the Boston Marathon in Boston, Massachusetts on Monday.

When the explosive is triggered, the pressure cooker itself, which is nearly always made of metal, explodes as well, adding to the spray of shrapnel, bomb experts in India said. Any metal container with a tight-fitting lid could be just as lethal, they said.

“The pressure cooker in itself is not a bomb,” explained Ajai Sahni, the executive director of the South Asia Terrorism Portal, a research group in Delhi. “It is just a packaging or a carrier.”

Forensic experts described the pressure cookers used in the Boston marathon blasts as generic, but  noted the marking “6L,” indicating six liters. Pressure cookers in the United States are most often measured in quarts, not liters.

There is no evidence at this point to suggest the planners of the Boston Marathon blasts had any ties to South Asia, defense experts here stressed.

A lot of terrorism training centers “are in this part of the world,” explained Mr. Sahni. “If I’m learning the rudiments of strapping together an IED, I might learn it from someone in this part of the world,” he said in referring to an improvised explosive device, with pressure cooker bombs being an obvious choice. On the other hand, he said, someone looking up how to make an IED on the Internet from anywhere in the world could find instructions on making a pressure cooker bomb.

As they have in the past in India, the pressure cooker bombs that exploded in Boston caused fatalities and devastating injuries, despite their homemade exterior:

“The resulting explosions sent metal tearing through skin and muscle,” Katherine Q. Seelye, Eric Schmitt and Scott Shane wrote in The New York Times, “destroying the lower limbs of some victims who had only shreds of tissue holding parts of their legs together when they arrived at the emergency room of Massachusetts General Hospital, doctors there said.”