An al Qaeda magazine containing bomb-making instructions helped the Boston Marathon bombers construct the deadly explosive devices that killed three people and wounded more than 260, according to a U.S. official close to the ongoing investigation.
The official confirmed that the disclosure about how the pressure cooker bombs were built came during hospital room exchanges between investigators and wounded bombing suspect Dzhokhar Tsarnaev.
The link between the bombers and the magazine is the main clue to emerge so far identifying the bombers as Islamist extremists seeking to wage jihad against the United States.
Tsarnaev said the English-language terrorist magazine Inspire provided details on the pressure cooker bombs, according to the official. The official commented after NBC News and Fox News Channel reported the detail.
Tsarnaev, who is recovering from multiple gunshot wounds in a Boston hospital, also said opposition to the wars in Iraq and Afghanistan, which the bombers viewed as a U.S. war against Islam, were a main motivation behind the Boston Marathon attack.
Officials continued to search for clues to any foreign affiliations between the two ethnic Chechens and terrorist groups in Russia and elsewhere.
Timothy Furnish, a specialist on Islamic terrorism and consultant to the U.S. military, said the surprising aspect of the case is that the Inspire link appears to have "caught government and some law enforcement officials by surprise."
"Inspire has long been publishing aritcles such as 'Make a Bomb in the Kitchen of Your Mom,'" Furnish said in an email. "Additional training by, say Chechen Muslim terrorist groups would provide further expertise, but is no long necessary for the aspiring jihadist in the United States or any other non-Muslim-majority country. There is a reason that the al Qaeda entity that publishes Inspire calls iteself 'al-Malahim Media' -- malahim means 'epic bloody struggles' in Arabic."
U.S. officials told the Washington Free Beacon shortly after the April 15 bombing that there were indications Inspire helped the bombers, specifically instructions contained in the first issue directing terrorists to use kitchen materials to make bombs.
The instructions included photos of a pressure cooker that was filled with gunpowder and designed to enhance its lethality by adding metal balls and nail.
The magazine said such devices could kill "tens" of people.
The Inspire article was contained in a section titled "Open Source Jihad: Make a Bomb in the Kitchen of Your Mom."
The report said jihadists in the United States and Europe should work inside those regions and copy the attacks of Army Maj. Nidal Hasan, the Fort Hood shooter, and Faisal Shahzad, the Islamist attacker and failed Times Square bomber.
Kitchen-made bombs can be constructed to avoid detection by bomb-sniffing dogs and also do not raise suspicions of security services.
"The pressurized cooker is the most effective method," the article said. "Glue shrapnel to the inside of the pressurized cooker then fill the cooker with inflammable material."
The article urged placing the device in a crowded area and to camouflage it.
The Tsarnaev brothers constructed at least three pressure cooker bombs and concealed them in backpacks that were remotely detonated using the battery of a remotely controlled toy.
In addition to the pressure cooker bomb, Issue 8 of Inspire contained detailed instructions on how to build a remote detonator from a remote-control motorcycle alarm.
A third Inspire, from May 2012, urged terrorists to attack sporting events or other large gatherings.
"This is done by targeting human crowds in order to inflict maximum human losses," the magazine said. "This is very easy since there are numerous such targets such as crowded sports arenas, annual social events, large international exhibitions, crowded market- places, sky-scrapers, crowded buildings ... etc."
Tsarnaev was arrested Friday after a massive police manhunt that included a shootout with police. Tsarnaev’s older brother Tamerlan was killed after being run over by his brother in a stolen SUV.
The two men have been charged by federal authorities with making a weapon of mass destruction and using explosive devices that result in death.
The weapon of mass destruction charge could result in the imposition of the death penalty.
News reports said the men obtained gunpowder for use in the bombs from fireworks purchased in Seabrook, N.H. on Feb. 6. The purchases included large "mortar" tubes used for fireworks displays.
Investigators said on Wednesday that the bombs were set off by electronic remote controls that required the person pressing the button to be within several blocks of the device, the Associated Press reported.
Police were able to identify the bombers from photographs taken immediately after the twin blasts by the fact that they were not visibly upset and were not urgently fleeing the scene.
An FBI criminal complaint made public earlier this week suggested the bombs were set off by the use of a cell phone, based on cell phone usage observed by the suspect in surveillance video around the time of the explosions.
Cell phones are used frequently in Afghanistan to detonate bombs. The triggering mechanism involves wiring the phone’s ringer to send a spark to an explosive, setting off a larger blast.