The Boston Marathon is the oldest annual, continuous-running marathon in the world, beginning more than a century ago.
It is held on Patriot's Day each year in Boston, attracting thousands of contestants and tens of thousands of spectators.
In 2013, Dzhokhar and Tamerlan Tsarnaev, two radical Chechen  brothers, detonated a pair of bombs near the finish line of the marathon. In response, the race was terminated, and thousands of runners were prevented from crossing the finish line. Three spectators were killed and 264 people were wounded.