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CNN's Joe Johns looks at the nature of the explosives used in the Boston Marathon blasts.
A suspicious letter containing ricin, a poison, was addressed to a senator, according to sources. CNN's Dana Bash reports.
RELATED STORY: Ricin scare rattles Washington
CNN's Wolf Blitzer reports the third victim in the Boston Marathon attack was a Boston University graduate student.
RELATED STORY: Sadness, resolve as Boston comes to grips with deadly marathon bombing
Patty Campbell says Monday's terror attack in Boston doesn't make any sense. Her daughter, Krystle, died in the bombing.
RELATED STORY: 'Boston will overcome,' mayor vows amid bombing investigation