CENTENNIAL, Colo. — Investigators on Saturday were working to find out what motivated a teenage gunman to enter his suburban Denver high school armed with a shotgun looking for a specific teacher a day earlier.
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 the strategic response by police on the eve of the Newtown massacre anniversary appears to have averted more bloodshed.
About a half hour after wounding the girl, Pierson was found dead in the school, apparently of a self-inflicted gunshot wound.
"I believe the shooter took his life because he knew he had been found," Arapahoe County Sheriff Grayson Robinson said.
Two other students were treated for minor injuries and released.
Robinson said Pierson, a senior who participated in debate activities, had had an altercation with the teacher, but the sheriff did not provide details or elaborate on a motive.
Authorities evacuated hundreds of students in an orderly procession — a demonstration of aggressive security measures developed by police and schools following the 1999 shooting at Columbine, some 8 miles west of Arapahoe High. All of the students there started school after Columbine and grew up in a security-conscious era.
After that tragedy, police across the country developed "active shooter" training in which responding officers rush toward gunfire — and step over bodies and bleeding victims if necessary — to stop the gunman.
"The first deputy sheriffs and police officers that were on scene immediately entered the school to engage the shooter if they could locate that individual and keep the other students safe," Robinson said.
"This kid, the officers went right to him literally within minutes," said Gov. John Hickenlooper. "That is a world of change from the way response used to happen."
Before Columbine, officials followed a contain-and-wait strategy in which arriving officers set up a perimeter to contain the situation, then wait for SWAT team members trained in military tactics to bring down the gunman.
Robinson said Pierson made no attempt to hide his weapon while entering the school from a parking lot and asking for the teacher by name.
When the teacher learned that he was being targeted, he left "in an effort to try to encourage the shooter to also leave the school," Robinson said. "That was a very wise tactical decision."
Students heard shots and cowered in classrooms while awaiting word on what had happened.
Senior Megan Jeffords, 18, was singing Christmas carols in the hall with her choir class when the shots rang out. A teacher rushed the 18 singers into a closet, where they huddled for more than half an hour.
Hours later, after Jeffords was reunited with her father, she was still visibly shaken and unable to talk much about what happened.
Jessica Girard was in math class when she said she heard three shots.
"Then there was a bunch of yelling, and then I think one of the people who had been shot was yelling in the hallway, ‘Make it stop,’" she said.
Two suspected Molotov cocktails were also found inside the school, the sheriff said. One detonated, though no one was injured.
The shooting came a day before the anniversary of the Newtown, Conn., attack in which a gunman killed 20 children and six adults at Sandy Hook Elementary School.
Since Columbine, Colorado has endured other mass shootings, including the killing of 12 people in a movie theater in nearby Aurora in 2012. But it was not until after the Newtown massacre that state lawmakers moved to enact stricter gun control laws.
Two Democratic lawmakers were recalled from office earlier this year for backing the laws, and a third recently resigned to avoid a recall election.
Associated Press writer Ivan Moreno in Denver contributed to this report.