[A man limps by after finishing the Boston Marathon. Today marks the 118th Boston Marathon; security presence has been increased this year, due to two bombs that were detonated at the finish line last year, killing three people and injuring more than 260 others. Photo by Andrew Burton/Getty Images.]

[A Wellesley College student gets a kiss from a runner as the field passes by in Wellesley, Mass. AP Photo/Mary Schwalm.]

[Two police officers look on from a rooftop near the start area of the 118th Boston Marathon in Hopkinton, Massachusetts. Photo by Alex Trautwig/Getty Images.]

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[Meb Keflezighi of the United States reacts after winning the Boston Marathon. Meb becomes the first American winner of the Boston Marathon since 1983. Photo by Jared Wickerham/Getty Images.]

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[Boston Mayor Martin Walsh places a wreath on Meb Keflezighi, of San Diego, Calif., after Keflezighi won the 118th Boston Marathon. AP Photo/Elise Amendola.]

[Rita Jeptoo of Kenya crosses the finish line in first place in the women's division with an official time of 2:28:57. Photo by Jared Wickerham/Getty Images.]

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[A competitor streaks along the course in the wheelchair division of the 118th Boston Marathon. AP Photo/Robert F. Bukaty.]

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[A father and son watch the scene at the starting line. Photo by Andrew Burton/Getty Images.]