As investigators comb the area near the finish line of the Boston Marathon searching for clues behind the two blasts Monday that killed three people and injured more than 170, they are focusing in on the pressure cooker used to build at least one of the bombs.

An unclassified security bulletin sent by the FBI Tuesday evening to law enforcement agencies across the country said that the bomb inside a shredded backpack found on the scene was an "improvised explosive device (IED)" made out of a common pressure cooker.

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ABC News has confirmed the device was made from a mid-size, Fagor brand pressure cooker, a Spanish line that sells around 50,000 of the units in the U.S. each year.

Pressure cooker bombs are well-known to the U.S. military, which has seen them used everywhere from Afghanistan to the streets of Nepal. The U.S. Department of Defense warns soldiers in its handbook on spotting roadside bombs to also be on the lookout for pressure cookers, a bomb made from the kitchen.

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The bombs are popular, according to experts, because they seal and can be filled with nails, ball bearings and projectiles, all of which were reportedly used in the Boston Marathon bombs.

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Walt Houston, a bomb expert who studies IEDs, says the pressure cooker bomb used in Boston appears basic but that does not mean the bombmaker is not sophisticated.

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"If he just wanted to make a point and blast and disrupt the marathon, he didn't have to add the shrapnel," Houston said.

Houston's point is echoed by a trauma surgeon with nine years of military experience who told ABCNews.com the bombmakers weren't looking to scare people, but intended to kill.

"That person or those people did everything they could to create a bomb that damaged and injured as many people as they possibly could," said former Navy surgeon Dr. Gary Schwendig, who now works at Scripps Health in San Diego.

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Schwendig said a bomb's shock wave alone was often enough to rupture blood vessels in victims' lungs and other organs in a way that proves fatal, and that debris picked up by explosions - fences, barricades, wood - tends to fly at people, severing limbs. Shrapnel packed into bombs is made to pepper victims with nails and metal ball bearings, severing veins and arteries.

"It's just one more way to hurt people," Schwendig said of the shrapnel Boston doctors said was built into the bombs. "A nail that is tumbling through the body - that nail is an-inch-and-a-half long and it's tumbling and it's going to cut right through that artery."

Still, most of the ball bearings and other pieces of shrapnel will probably remain with the victims for the rest of their lives, he said. Removing the fragments could cause more damage and increase the likelihood of infection, so trauma doctors often leave the shrapnel where it is.

Many victims of the Boston Marathon bombings face possible amputations in the coming days and months, hospitals reported.

ABC News' Sydney Lupkin contributed to this report.