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Charles Krupa, ASSOCIATED PRESS
Medical workers aid injured people at the finish line of the 2013 Boston Marathon following an explosion in Boston, Monday, April 15, 2013. Two explosions near the finish of the Boston Marathon on Monday, killing at least two people, injuring over 20 others. (AP Photo/Charles Krupa)

Many people and news organizations first learned of the explosions at today's Boston Marathon via Twitter.

What follows is a roundup of reactions on Twitter, including photos and expressions of faith from people who are struggling to understand the horror associated with the explosions.