i have a student down in the athletic hall. two shotgun shells on the ground here. reporter: the high school is still a crime scene. at this time we have one student down and they found shotgun shells. reporter: police say the gunman identified as 18-year-old karl pierson shot one student before turning the gun on himself, taking his own life. it was pretty scary. there were two shots by my classroom. so we heard the screaming. reporter: pierson appears to have been seeking revenge against a specific faculty member because of what police call a confrontation or disagreement. witnesses saw pierson enter arapahoe high school carrying a shotgun. he made no attempt to hide it. i saw him. he was kind of running military toward the building. i alerted everybody in the building. and that s when i heard two big bangs, boom, pop.