Boston Marathon Bombing

Boston Bomber(s) Not Yet Identified

Three dead, at least 176 injured

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Hundreds of U.S. law enforcement agents are racing to gather and analyze every shred of evidence from Monday's deadly Boston Marathon bombing for a hint of the attacker, but the morning after three were killed and nearly 150 more injured in twin explosions, no one has been taken into custody.

Overnight a tip about possible explosives led federal agents to search an apartment on the fifth floor of a building in the Boston suburb of Revere. Agents later told residents there was nothing to worry about, but the quick and overwhelming law enforcement response underscored the urgency of the FBI's effort to track and stop the people responsible.