(Reuters) - A U.S. jury on Friday sentenced Boston Marathon bomber Dzhokhar Tsarnaev to death for helping to carry out the 2013 attack that killed three people and injured 264 at the world-renowned race, taking 15 hours to reach a decision. The federal jury chose death by lethal injection for Tsarnaev, 21, over its only other option: life in prison without possibility of release. The same panel last month found the ethnic Chechen guilty of placing a pair of homemade pressure-cooker bombs at the race's crowded finish line on April 15, 2013, as well as fatally shooting a policeman. The bombing was one of the highest-profile attacks on U.S. soil since Sept. 11, 2001. During 10 weeks of testimony, the jury heard from about 150 witnesses, including people whose legs were torn off by the shrapnel-filled bombs. William Richard, the father of bombing victim Martin Richard, described the gut-wrenching decision to leave his 8-year-old son to die of his wounds so ...