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The blasts occurred shortly before 3 p.m. Monday near the finish line on Boylston Street, transforming a prized Boston tradition into a gruesome scene. Doctors said they had to amputate limbs and pull metal shards from the injured.

The victims who died have been identified as Martin Richard, 8; Krystle Campbell, 29; and Chinese citizen Lu Lingzi, who had been studying mathematics and statistics as a graduate student at Boston University.

Many of the injured remained in hospitals, seven with critical injuries.

Earlier Thursday, Homeland Security Secretary Janet A. Napolitano testified before the House Homeland Security Committee that it was unclear whether those who planted the bombs at the marathon were foreign or domestic terrorists.