CENTENNIAL, Colo. (AP) — 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 contributed to this report.

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  • A father picks up his daughter at a church where students from nearby Arapahoe High School were evacuated to after a shooting on the Centennial, Colo., campus Friday, Dec. 13, 2013. Arapahoe County Sheriff Grayson Robinson said the shooter shot two others at the school, before apparently killing himself.(AP Photo/Brennan Linsley)

  • School Shooting, Grayson Robinson

    Media cover a briefing by Arapahoe County Sheriff Grayson Robinson Saturday, Dec. 14, 2013, at Arapahoe High School in Centennial, Colo., where 18-year-old shooting suspect Karl Halverson Pierson shot student Claire Davis, 17,on Friday, then turned the gun on himself. AP Photo/Ed Andrieski)

  • School Shooting, Grayson Robinson

    Arapahoe County Sheriff Grayson Robinson holds a picture of Claire Davis the 17-year-old student that was shot during a briefing Saturday, Dec. 14, 2013, at Arapahoe High School in Centennial, Colo. Davis was shot by 18-year-old shooting suspect Karl Halverson Pierson at the school on Friday before he took his own life. (AP Photo/Ed Andrieski)

  • Arapahoe High School

    Roses and a sign of support are woven into a cyclone fence around a tennis court at Arapahoe High School in Centennial, Colo., on Saturday, Dec. 14, 2013. The school was the scene of a shooting on Friday that left a student gunman dead and two other students injured. (AP Photo/David Zalubowski)

  • School Shooting, Sharon Adams

    Sharon Adams looks through a 2013 Arapahoe High School yearbook at her home near the school in Centennial, Colo., on Saturday, Dec. 14, 2013. The Adams daughter is a cheerleader at the school and escaped unharmed. Pierson critically wounded one student, before apparently killing himself Friday at Arapahoe High School as police moved in. (AP Photo/Ed Andrieski)

  • School Shooting

    The front door at the Highlands Ranch, Colo., home of the 18-year-old shooting suspect Karl Halverson Pierson is covered with plywood on Saturday, Dec. 14, 2013, after the police searched the home. Pierson critically wounded one student, before apparently killing himself Friday at Arapahoe High School as police moved in. (AP Photo/Ed Andrieski)

  • Arapahoe County Sheriff's Department deputy

    A deputy of the Arapahoe County, Colo., Sheriff's Department walks next to crime scene tape in front of a doorway to Arapahoe High School in Centennial, Colo., on Saturday, Dec. 14, 2013. The school was the scene of a shooting on Friday that left the student gunman dead and two other students injured. (AP Photo/David Zalubowski)

  • Arapahoe High School, Arapahoe Sheriff's deputy

    A deputy from the Arapahoe County, Colo., Sheriff's Department gets in his patrol car parked outside the west entrance to Arapahoe High School, the scene of a shooting on Friday, in Centennial, Colo., early on Saturday, Dec. 14, 2013. Investigators 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. The 18-year-old shooter critically wounded a 15-year-old student and was later found dead in the school, apparently of a self-inflicted gunshot wound. (AP Photo/David Zalubowski)

  • Sean Sweeney

    Sean Sweeney, a runner who lives near Arapahoe High School in Centennial, Colo., pauses to say a prayer for the victims and the gunman of a shooting at the school during his early morning run on Saturday, Dec. 14, 2013. The school was the scene of a shooting on Friday that left the student gunman dead and two other students injured. (AP Photo/David Zalubowski)

  • Chris Davis

    Chris Davis, a senior who plays basketball at Arapahoe High School, talks to reporters as he places a donation box for the victim of the shooting at the school located in Centennial, Colo., on Saturday, Dec. 14, 2013. Investigators 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. The 18-year-old shooter critically wounded a 15-year-old student and was later found dead in the school, apparently of a self-inflicted gunshot wound. (AP Photo/David Zalubowski)

  • Michael Vartuli, Chris Davis

    Michael Vartuli, left, and Chris Davis, both seniors at Arapahoe High School, hang a sign asking for donations for the victim of the shooting at the school in Centennial, Colo., early on Saturday, Dec. 14, 2013. Investigators 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. The 18-year-old shooter critically wounded a 15-year-old student and was later found dead in the school, apparently of a self-inflicted gunshot wound. (AP Photo/David Zalubowski)

  • School Shooting

    Police walk up to search the Highlands Ranch, Colo., home of the 18-year-old shooting suspect Karl Halverson Pierson who shot a student at Arapahoe High School in Centennial, Colo., on Friday, Dec. 13, 2013, then turned the gun on himself. (AP Photo/Ed Andrieski)

  • School Shooting

    Police used a dog to do an initial search at the Highlands Ranch, Colo., home of the 18-year-old shooting suspect Karl Pierson who shot a student at Arapahoe High School in Centennial, Colo., on Friday, Dec. 13, 2013, then turned the gun on himself. (AP Photo/Ed Andrieski)

  • School Shooting

    Police gather at the Highlands Ranch, Colo., home of the 18-year-old shooting suspect who shot two students at Arapahoe High School in Centennial, Colo., on Friday, Dec. 13, 2013. The police are expected to search the house Friday night. (AP Photo/Ed Andrieski)

  • Allie Zadrow, Liz Reinhardt

    Freshman Allie Zadrow, right, is hugged by a friend Liz Reinhardt at a church after a shooting at nearby Arapahoe High School in Centennial, Colo., on Friday, Dec. 13, 2013. Students from the school were evacuated to the church. Arapahoe County Sheriff Grayson Robinson said the shooter shot two others at the school, before apparently killing himself.(AP Photo/Brennan Linsley)

  • A parents picks up her daughter at a church where students from nearby Arapahoe High School were evacuated to after a shooting on the Centennial, Colo., campus Friday, Dec. 13, 2013. Arapahoe County Sheriff Grayson Robinson said the shooter shot two others at the school, before apparently killing himself.(AP Photo/Brennan Linsley)

  • School Shooting

    Parents pick up their daughter at a church where students from nearby Arapahoe High School were evacuated to after a shooting on the Centennial, Colo., campus Friday, Dec. 13, 2013. Arapahoe County Sheriff Grayson Robinson said the shooter shot two others at the school, before apparently killing himself.(AP Photo/Brennan Linsley)

  • Allie Zadrow, Liz Reinhardt

    Freshman Allie Zadrow, right, is hugged by a friend Liz Reinhardt at a church after a shooting at nearby Arapahoe High School in Centennial, Colo., on Friday, Dec. 13, 2013. Students from the school were evacuated to the church. Arapahoe County Sheriff Grayson Robinson said the shooter shot two others at the school, before apparently killing himself.(AP Photo/Brennan Linsley)

  • Courtney Leypoldt

    Courtney Leypoldt, left, is hugged by a family member as she is picked up at a church after a shooting at nearby Arapahoe High School in Centennial, Colo., on Friday, Dec. 13, 2013. Students from the school were evacuated to the church. Arapahoe County Sheriff Grayson Robinson said the shooter shot two others at the school, before apparently killing himself.(AP Photo/Brennan Linsley)

  • School Shooting

    A student stares out a bus window as it arrives at a church to be reunited with family after students from nearby Arapahoe High School were evacuated their after a shooting at the school in Centennial, Colo., Friday, Dec. 13, 2013. Arapahoe County Sheriff Grayson Robinson said the shooter shot two others at the school, before apparently killing himself.(AP Photo/Brennan Linsley)

  • A woman screams as she arrives at a church where students from nearby Arapahoe High School were evacuated to after a shooting on the Centennial, Colo., campus Friday, Dec. 13, 2013. Arapahoe County Sheriff Grayson Robinson said the shooter shot two others at the school, before apparently killing himself.(AP Photo/Brennan Linsley)

  • Jenni Meyers, Mary Meyers, Julie Meyers

    Senior Jenni Meyers, center, is hugged by her sister Mary as they leave a church with their mother Julie after they were reunited after a shooting at nearby Arapahoe High School in Centennial, Colo., on Friday, Dec. 13, 2013. Arapahoe County Sheriff Grayson Robinson said the shooter shot two others at the school, before apparently killing himself.(AP Photo/Brennan Linsley)

  • School Shooting

    Students comfort each other outside of Arapahoe High School after a shooting on the campus in Centennial, Colo., on Friday, Dec. 13, 2013. Arapahoe County Sheriff Grayson Robinson said Friday that the shooter, who died, was a student at the high school. He said another student was wounded when he confronted the armed student and was shot. (AP Photo/Ed Andrieski)

  • School Shooting

    Sheriffs deputies stand guard over students after they were evacuated to the track and football field at Arapahoe High School in Centennial, Colo., on Friday, Dec. 13, 2013, where a student shot at least one other student at a Colorado high school Friday before he apparently killed himself, authorities said. The shooter entered Arapahoe High School in a Denver suburb armed with a shotgun and looking for a teacher he identified by name, said Arapahoe County Sheriff Grayson Robinson. (AP Photo/Ed Andrieski)

  • School Shooting

    Sheriffs deputies stand guard over students after they were evacuated to the track and football field at Arapahoe High School in Centennial, Colo., on Friday, Dec. 13, 2013, where a student shot at least one other student at a Colorado high school Friday before he apparently killed himself, authorities said. The shooter entered Arapahoe High School in a Denver suburb armed with a shotgun and looking for a teacher he identified by name, said Arapahoe County Sheriff Grayson Robinson. (AP Photo/Ed Andrieski)

  • School Shooting

    A sheriff deputy talks on his radio at Arapahoe High School in Centennial, Colo., on Friday, Dec. 13, 2013, where a student shot at least one other student at a Colorado high school Friday before he apparently killed himself, authorities said. The shooter entered Arapahoe High School in a Denver suburb armed with a shotgun and looking for a teacher he identified by name, said Arapahoe County Sheriff Grayson Robinson. (AP Photo/Ed Andrieski)

  • School Shooting

    Students comfort each other at Arapahoe High School in Centennial, Colo., on Friday, Dec. 13, 2013, where a student shot at least one other student at a Colorado high school Friday before he apparently killed himself, authorities said. The shooter entered Arapahoe High School in a Denver suburb armed with a shotgun and looking for a teacher he identified by name, said Arapahoe County Sheriff Grayson Robinson. (AP Photo/Ed Andrieski)

  • In this still image taken from video provided by Fox 31 Denver, students gather just outside of Arapahoe High School as police respond to reports of a shooting at Arapahoe High School in Centennial, Colo. Friday, Dec. 13, 2013. Colorado division of emergency management spokeswoman Micki Trost said her director went to the school and their weren't any more immediate details. (AP Photo/KDVR) MANDATORY CREDIT

  • In this still image taken from video provided by Fox 31 Denver, police respond to reports of a shooting at Arapahoe High School in Centennial, Colo. Friday, Dec. 13, 2013. Colorado division of emergency management spokeswoman Micki Trost said her director went to the school and their weren't any more immediate details. (AP Photo/KDVR) MANDATORY CREDIT

  • In this still image taken from video provided by Fox 31 Denver, students gather just outside of Arapahoe High School as police respond to reports of a shooting at Arapahoe High School in Centennial, Colo. Friday, Dec. 13, 2013. Colorado division of emergency management spokeswoman Micki Trost said her director went to the school and their weren't any more immediate details. (AP Photo/KDVR) MANDATORY CREDIT

  • In this still image taken from video provided by Fox 31 Denver, police respond to reports of a shooting at Arapahoe High School in Centennial, Colo. Friday, Dec. 13, 2013. Colorado division of emergency management spokeswoman Micki Trost said her director went to the school and their weren't any more immediate details. (AP Photo/KDVR) MANDATORY CREDIT

  • Police respond to the shooting at Arapahoe High School in Centennial, Colo. Friday, Dec. 13, 2013.

  • The FBI is seen responding to a shooting at Arapahoe High School in Centennial, Colo. Friday, Dec. 13, 2013.

  • Police respond to the shooting at Arapahoe High School in Centennial, Colo. Friday, Dec. 13, 2013.

  • Students were escorted out Arapahoe High School in Centennial, Colo. with their hands up after a shooting broke out on Friday, Dec. 13, 2013.

  • Police respond to the shooting at Arapahoe High School in Centennial, Colo. Friday, Dec. 13, 2013.

  • Arapahoe County Sheriff Grayson Robinson speaks during a media briefing about the shooting at Arapahoe High School in Centennial, Colo. Friday, Dec. 13, 2013.

  • Colorado Gov. John Hickenlooper speaks at a media briefing about the shooting at Arapahoe High School in Centennial, Colo. Friday, Dec. 13, 2013.

  • Colorado Gov. John Hickenlooper is seen shortly after a press conference at Arapahoe High School in Centennial, Colo. Friday, Dec. 13, 2013.

  • Arthur Blessitt of Littleton who is <a href="http://www.guinnessworldrecords.com/world-records/1/longest-ongoing-pilgrimage" target="_blank">listed in the Guinness World Records</a> for the world's longest walk carrying a wooden cross around the world was nearby after the the shooting at Arapahoe High School in Centennial, Colo. on Friday, Dec. 13, 2013.

  • Buses carrying students from Littleton Public Schools leave after a shooting occurred at Arapahoe High School in Centennial, Colo. on Friday, Dec. 13, 2013.

  • CO SCHOOL SHOOTING 2

    UPDATES WITH DETAILS OF THE EVENTS: Map locates Arapahoe High School shooting. ; 1c x 3 inches; 46.5 mm x 76 mm;