Federal investigators said the bombs that exploded near the finish line at the Boston Marathon Monday afternoon were pressure cookers filled with nails, bb's and other shrapnel designed to inflict as much damage as possible.
The bombs were then placed in large, nylon bags dropped along the race's route.
More information is now being learned about the three known victims that died in the explosions. A Chinese newspaper said a woman studying at Boston University was one of those killed.
Also killed in the double bombings was Krystle Campbell, 29, and Martin Richards, 8, who was watching his father race.
On Tuesday night, thousands of people gathered in the boy's hometown of Dorchester for a vigil.
President Barack Obama will pay his respects at an interfaith vigil in Boston on Thursday - his fourth trip to a city trying to come to grips with a violent attack.
"This is a heinous and cowardly act and given what we know now about what took place," Obama said. "The FBI is investigating it as an act of terrorism."
The FBI is sending pieces of the bomb to its headquarters in Quantico, VA, where they'll try to reconstruct it, vowing to go to ends of the earth to find whomever is responsible.
They're urging people to come forward if they heard anyone mention targeting the marathon, showed a suspicious interest in explosives, or if they spotted someone on race day carrying a heavy, dark bag.
More information about the bombings is expected to be released Wednesday afternoon.
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