President Barack Obama underlined Tuesday that investigators do not know if the twin bombing the day before that killed three people and wounded more than 170 was carried out by an international organization, a domestic group or a "malevolent individual." There has been no claim of responsibility.
A person briefed on the investigation told The Associated Press that the explosives were fashioned out of pressure cookers and packed with shards of metal, nails and ball bearings to inflict maximum carnage.
The relative ease of constructing such bombs and the powerful punch they deliver has made them attractive to insurgents and Islamic extremists, particularly in South Asia. They have turned up in past bombing plots by Islamic extremists in the West, including a plan by a U.S. soldier to blow up a restaurant frequented by fellow soldiers outside Fort Hood, in Texas. One of the three devices used in the May 2010 Times Square attempted bombing was a pressure cooker, according to a joint FBI and Homeland Security intelligence report issued in July 2010.
Al-Qaida's branch in Yemen gave a detailed description on how to make a pressure cooker bomb in the 2010 first issue of "Inspire," its magazine that only appears online, in a chapter titled "Make a bomb in the kitchen of your mom."
"The pressurized cooker is the most effective method" for making a simple bomb, the article said, describing how to fill the cooker with shrapnel and gunpowder and to create a detonator using the filament of a light bulb and a clock timer.
"Inspire" magazine has a running series of such training articles called "Open Source Jihad," which the group calls a resource manual for individual extremists to carry out attacks against the enemies of jihad, including the U.S. and its allies.The magazine is targeted heavily at encouraging "lone wolf" jihadis.
An issue last year reprinted an older article by a veteran Syrian jihadi Abu Musab al-Souri addressing would-be jihadis proposing a long list of possible targets for attacks, among them "crowded sports arenas" and "annual social events."
Notably, Army Pfc. Naser Jason Abdo, who was convicted and sentenced to life in prison last year for the Fort Hood restaurant bombing plot, was discovered to have a copy of the "How to build a bomb in the kitchen of your mom" article, according to the FBI. Investigators found bombmaking materials in his hotel that included a pressure cooker and gunpowder, according to testimony at his trial.
The SITE Monitoring Service, a U.S. independent group tracking militant messaging online, noted that Islamic extremists are not the only ones paying attention to the al-Qaida magazine: White supremacists have also circulated copies on their web forums. They found "Inspire" and "other al-Qaida manuals beneficial for their strategies," it said.
Over the course of 10 issues the past three years, "Inspire" has given detailed instructions with diagrams and photos on how to use automatic weapons, produce remote control detonators, set fire to a building or create forest fires. In the most recent issue, put out in March, it described how to set fire to a parked vehicle and how to cause road accidents with oil slicks on a road or tire-bursting spikes.
The chapters, including the one on pressure cooker bombs, were compiled into a booklet titled "The Lone Mujahed Pocketbook," released on Islamic militant web sites in March, according to SITE.
Al-Qaida's Yemeni branch, known as al-Qaida in the Arabian Peninsula, has repeatedly tried to carry out direct attacks on U.S. soil, once by dispatching the would-be 2009 Christmas bomber of a U.S. jet whose attack failed when the explosives hidden in his underwear failed to go off and then the following year by trying to mail explosives to the U.S. in packages that were intercepted.
The pressure cooker bomb's most frequent use seems to be in Afghanistan, Pakistan, Nepal and India in attacks against police or the public. This year, local press reports in Pakistan have reported several such bombs found planted on streets, including in the city of Karachi, where multiple militant groups operate.
In 2010, suspected militants attacked the U.S.-based Christian aid group World Vision in northwestern Pakistan, killing six Pakistani employees with a remotely detonated pressure cooker bomb.
That same year, the U.S. Department of Homeland Security put out a warning about such explosives, noting their frequent use in South Asia.
"The presence of a pressure cooker in an unusual location such as a building lobby or busy street corner should be treated as suspicious," it said.
AP correspondents Cassandra Vinograd and Paisley Dodds in London and Eileen Sullivan in Washington contributed to this report.