the boston marathon takes place on patriot's day each year, with more than 30,000 competitors and thousands of spectators lining the streets. it shocked the world when this peaceful race ended in tragedy on april 15, 2013. two chechen brothers, dzhokhar and tamerlan tsarnaev, aged 19 and 26, exploded two pressure cooker bombs near the finish line, killing three people and injuring 264. tamerlan was killed evading arrest. when dzhokhar was caught, he claimed the attack was motivated by islamic beliefs and revenge on the u.s. for the afghanistan and iraq wars.