CENTENNIAL, Colo. — Investigators were looking Saturday at whether revenge motivated a teenage gunman to enter his suburban Denver high school armed with a shotgun looking for a specific teacher.

Quick-thinking students at Arapahoe High School on Friday alerted the targeted educator, who quickly left the building. The 18-year-old shooter — identified by authorities as Karl Pierson — critically wounded a 15-year-old student, but strategic response by police on the eve of the Newtown massacre anniversary appears to have averted more bloodshed.