Police have arrested a 26-year-old man in the shooting death of 9-year-old Oscar Fuentes in his family's Columbia Heights apartment and authorities described him as an associate of the MS-13 street gang who was trying to rob people in the neighborhood. Police Chief Cathy L. Lanier said the suspect, Josue Pena, had followed Oscar's family into their dilapidated apartment building when he couldn't rob them outside. Pena, of no fixed address, then shot through the door. Authorities suggested that Oscar was looking through the peephole of the door when he was killed. ... Lanier wouldn't say why Pena decided to...