BOSTON (CBS/AP) – The two bombs used in the Boston Marathon bombings were improvised explosive devices (IEDs) made to look like discarded property, according to CBS News.

Correspondent Bob Orr reported Tuesday the bombs were made of low grade explosives, laced with shrapnel, ball bearings and nails.

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A law enforcement source told CBS News that one of the devices, found near the Starbucks on Boylston Street, appears to have been placed in a metal pressure cooker, which is a kitchen pot with a locked down top.  That device had been placed in a nylon bag or backpack, the source said.

Investigators also found pieces of an electronic circuit board possibly indicating a timer was used in the detonation.

It’s still not known if the bombs were placed in garbage cans before the explosions.

Massachusetts General Hospital surgeon Dr. George Velmahos told reporters Tuesday most of the injuries he has seen involve the victims’ lower extremities.

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“This bomb was obviously placed low on the ground, therefore lower extremity injuries are to be expected,” he said.

Four patients at MGH have had limbs amputated. Nails and metal fragments were found in many of the wounded, Velmahos said.

Similar pressure-cooker explosives have been used in Afghanistan, India, Nepal and Pakistan, according to a July 2010 joint FBI and Homeland Security intelligence report. Also, one of the three devices used in the May 2010 Times Square attempted bombing was a pressure cooker, the intelligence report said.

“Placed carefully, such devices provide little or no indication of an impending attack,” the report said.

The Pakistani Taliban, which claimed responsibility for the 2010 attempt in Times Square, has denied any role in the Boston Marathon attack.

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