STEVE GORMAN INVESTIGATORS sifted through a bloody crime scene and pored through surveillance video, social media and eyewitness accounts as they sought clues to what drove a 15-year-old boy to go on a deadly shooting spree at his Detroit-area high school. The young suspect, whose name was withheld by officials because he is a minor, opened fire on Tuesday with a semi-automatic handgun his father had purchased four days earlier, killing three fellow Oxford High School students. A teacher and seven other students were wounded, some critically, police said. Oakland County Sheriff Michael Bouchard told a news briefing hours after…