re: 2 Explosions at Boston Marathon - Updated w/Suspects Pictures *Warning Graphic*

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One of the deadly Boston Marathon bombs was apparently placed inside a metal pressure cooker and hidden in a backpack, a federal law enforcement official said Tuesday.

A timer was likely used to activate one of the bombs, rather than a cell phone, another law enforcement official said.

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The explosives themselves were small, and initial tests showed no C-4 or other high-grade explosive material -- suggesting the packages used in the attack were crude devices, a federal law enforcement official in the intelligence community said.

Based on the bombs' effects, the devices could have been small enough to be concealed in small bags or boxes, a law enforcement official said. The smoke was consistent with a "low-velocity improvised explosive mixture, perhaps flash powder or sugar chlorate mixture," the official said

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Two doctors overseeing treatment of the injured believe the explosive devices contained nails or similar objects.

Many patients have severe wounds "related to the blast effect of the bomb as well as small metallic fragments that entered their bod," including "pellets" and "nail-like objects," said Dr. George Velmahos, head of trauma care at Massachusetts General Hospital.

A variety of sharp objects were found inside the patients bodies, he said, adding that the bombs probably contained multiple metallic fragments.

Asked whether what was found in the patients' bodies could have come from nearby objects that exploded in the blast, Velmahos said he believes the materials were likely part of the explosive devices.

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A law enforcement official in Boston said investigators "have a number of active leads and some good early progress in the forensics analysis."

There were no credible threats ahead of the race, a state government official said.

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

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