Boston Marathon bombs were made from 6-liter pressure cookers crammed with shards of metal, nails and ball bearings and stashed in black backpacks, police sources revealed today.

The two bombs killed three people and injured at least 176 at the world’s oldest marathon race on Monday.

The bombs have “frequently” been used in Afghanistan, India, Nepal and Pakistan, according to a 2010 Homeland Security Department pamphlet – hinting at the origins of the bombers behind the worst terrorist atrocity in the U.S. since 9/11.

When the devices exploded near the crowded Boston Marathon finish line around 2.50 p.m. on Monday, victims suffered as many as 40 shrapnel wounds each and at least 10 people needed amputations.

The bombs used to kill and maim are believed to have contained black powder or gunpowder as the explosive, and information on how to make such a bomb is available on the internet, experts said. The devices were then left at the scene to look like discarded property, CBS News reported.

Boston Marathon bombs were made from 6-liter pressure cookers crammed with shards of metal, nails and ball bearings and stashed in black backpacks

Investigators have also found pieces of an electronic circuit board which could indicate a timer was used in the detonation.

Although no group has yet claimed responsibility for the attacks, similar devices were used in the failed 2010 attempt to bomb Times Square by Faisal Shahzad, who admitted he had undergone bomb-making training at a militant Islamist faction camp in Pakistan.

A pressure-cooker bomb is also a preferred weapon of al-Qaeda and listed as the “most effective” weapon of jihad, according to an English-language terror magazine called Inspire, in an article entitled “How to Build a Bomb in the Kitchen of Your Mom”.

Another article in Inspire last year listed “the most important enemy targets” for jihadists in America – and included sporting events, CNSNews noted.

The aim should be to target “human crowds in order to inflict maximum human losses”, a terrorist known as Abu Musab al-Suri wrote.

“This is very easy since there are numerous such targets such as crowded sports arenas, annual social events, large international exhibitions… etc.”