Prosecutors: Gun used to kill Officer Sean Collier . A man who loaned a gun used by the Boston Marathon bombers to kill a police officer was sentenced Tuesday to the 17 months he has already served and apologized, saying his actions were "dumb." I was young, dumb, and thought I could outsmart everyone," 22-year-old Stephen Silva told a judge in U.S. District Court. He pleaded guilty to gun and heroin distribution charges last year.