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Boston Marathon bombing: '6L' mark, circuit boards may be key clues (+video)

Investigators have scoured the crime scene of the Boston Marathon bombing for every possible physical clue. A crucial task is to collect as many bomb fragments as possible, to track where the parts came from and who bought such items.

FBI seeks photos of Boston Marathon attack
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Federal agents are quickly determining how the Boston Marathon bombing was carried out, but the question of who is responsible remains a mystery as the investigation into the deadly attack scours a massive crime scene for answers.

Under the direction of the FBI, more than 1,000 law enforcement officers are going over every inch of a large portion of Boston’s Back Bay in an effort to find every scrap that might possibly represent a physical clue, said FBI Special Agent in Charge Richard DesLauriers at a Tuesday evening briefing. They’re interviewing every eye witness they can find and matching evidence with existing law enforcement databases.

Both of the bombs were packed in heavy, dark bags, probably duffel bags or backpacks, said Mr. DesLauriers. He asked anyone who had seen a person struggling with such a load near the marathon's finish line on April 15, or anyone who had heard a friend, relative, neighbor, or coworker talk about targeting the iconic race in any way, to call the FBI.

“Someone knows who did this. Cooperation from the community will play a crucial role in this investigation,” said DesLauriers.

Two blasts ripped through the Back Bay crowd seconds apart on Monday just before 3 p.m. Early indications are that the bombs had a basic explosive element, such as gunpowder or residue from match heads, placed inside common pressure cookers. Nails or ball bearings were added to create shrapnel.

Pictures from a law enforcement bulletin obtained by the Associated Press and Reuters showed pictures of mangled pressure cooker scraps, scraps of black nylon, a circuit board, and a battery with wires attached.


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