CENTENNIAL — A student carried a shotgun into Arapahoe High School, asked where to find a specific teacher and then opened fire on Friday, Arapahoe County Sheriff Grayson Robinson said. He shot a fellow student in the head before apparently killing himself.

A 15-year-old girl was reported in critical condition after undergoing surgery. Two other students were treated and released from a hospital for non-gunshot injuries.

The gunman, identified as 18-year-old Karl Pierson, was found dead inside a classroom from an apparent self-inflicted gunshot wound, Robinson said. Authorities believe he acted alone.

Robinson said authorities are investigating reports that Pierson may have been motivated by revenge against the teacher following a disagreement.

Local and federal investigators were at the gunman’s house Friday night along with a bomb squad. They also searched his father’s Denver home.

Fellow classmates described the gunman as a bright student and a gifted debater whose family attended Bible study meetings. His body remained inside the school Friday night.

Hundreds of students fled in terror from the school after the shooting began and many others huddled into the corners of their locked-down, darkened classrooms, where they waited for more than an hour for police to lead them outside.

“We were all just sitting there staying quiet and praying,” said 15-year-old Jessica Girard, who was in math class when she heard three loud bangs.

Outside the locked classroom door Jessica heard someone walk by saying. “It hurts. It hurts. Make it stop.”

The school, which has roughly 2,100 students, is at the corner of South University Boulevard and Dry Creek Road in Centennial.

As students left the school after the shooting, many held their hands in the air or on their heads, and police officers patted them down. They were taken by bus to Shepherd of the Hills church, at 7691 S. University Boulevard, or Euclid Middle School, at 777 W. Euclid Ave. Parents of Arapahoe High students were asked to go to the church or the school as well to reunite with their children.

“The school was evacuated very slowly, very deliberately and very meticulously,” Robinson said. “We wanted to ensure that all of our students were safe.”

Robinson said the first 911 call on the shooting came in at 12:33 p.m.

The shooter carried the shotgun openly as he entered the school’s west doors, making no attempt to hide his weapon, Robinson said. Once inside, the student asked where he could find a specific teacher, Robinson said. Alerted to the situation, the teacher quickly left the school, something Robinson praised as “the most important tactical decision that could be made.”

“He took himself away from the school in an effort to try to encourage the student to move with him,” Robinson said.

The shooter, though, remained in the school, where he shot a female student he came across.

Dozens of sheriff’s deputies and SWAT teams converged on the school, but the school’s resource officer was already in pursuit of the shooter. According to police scanner recordings, the deputy ran to the library, where he saw smoke, and then into the athletic hall. There he found the wounded girl.

“I have a student in the athletic hall,” he said into his police radio. “She is bleeding pretty bad.”

An ambulance arrived within minutes to take the girl to Littleton Adventist Hospital, where she underwent surgery Friday afternoon.

More officers swarmed into the school, “to engage the shooter if they could locate the individual,” Robinson said.

Once inside, Robinson said officers found another student covered in blood, but she had not been shot. That girl and a third student were taken to Swedish Medical Center in Englewood and treated for non-gunshot injuries. Both students were released from the hospital later on Friday.

Further inside the school, deputies found the suspected gunman dead in a classroom. Robinson estimated it took the school resource officer about five minutes to find his body.

Robinson said sheriff’s deputies did not fire their weapons.

Officers also found two Molotov cocktails inside the school, Robinson said, one of which had been ignited. Robinson said investigators had already contacted the gunman’s parents.

At an afternoon news conference, Gov. John Hickenlooper lamented the “all-too familiar sequence of gunshots” at a Colorado school, but praised law enforcement and first-responders for their quick action.

“In this case we saw the incredible training and preparation of our first responders,” said Hickenlooper, who met with the family of the critically injured girl Friday afternoon.

Students and teachers, too, acted quickly after the sounds of shotgun blasts echoed down the school’s hallways, a sign of how much has changed since the tragic shootings at nearby Columbine High School, which occurred when most current students at Arapahoe were just toddlers.

Saturday is the one-year anniversary of the Sandy Hook school shooting in Connecticut.

Arapahoe High School is about eight miles east of Columbine High School, where two students killed 12 students and one teacher in 1999. The school is about 15 miles southwest of the Aurora movie theater where a gunman shot 70 people, killing 12, in July 2012.

Seniors Carl Schmidt and Brendon Mendelson were in yoga class at Arapahoe High when the shooting began. Their teacher hurried them promptly away from the door and into a closet.

“You always had the sense that nothing bad would happen to you,” Schmidt said.

Student Justin Morrall said students had been trained to move to the corners of classrooms where they would not be visible. Morrall said he heard screams when the shots were fired, but his classroom fell silent.

“Then we went into the drill positions,” Morrall said.

The orderliness, though, belied the terror in the classrooms.

Students said they hid in the corners of dark classrooms until they were led out by police. Adam Jones, whose sister goes to high school, said students were still locked in rooms about an hour after the shooting began and were “very scared.”

A teacher wrote in a text message from inside the school: “lockdown-super scary.”

Will Torres, a sophomore, was in weight class when he heard two gun blasts. The class’s teacher slammed shut the door to the weight room and told students to get on the floor.

“My heart started sinking,” Torres said. “I though about my family and other school shootings. … Some kids were crying. I was so scared.”

Torres said he was in the weight room for about 45 minutes then evacuated to the church nearby.

Nich Herzog, a senior at Arapahoe High School, stayed home from school because he was sick. He said he learned about the shooting after getting messages from friends in other schools asking if he was OK.

Herzog then started texting his friends at the school to find out what was happening.

“A lot of them are scared,” Herzog said. “One of my friends was right next to someone who got shot and she’s really shaken up. They are really scared and they want to get out.”

Inside her classroom, Girard said students were sitting quietly and praying.

“I was thinking I was going to die and I was never going to see my family again and I was praying that they knew how much I loved them,” Jessica said.

Parents dashed to the school to find their kids.

Earlier in the day, Chris Foster’s daughter, Devan, sent him a text message saying, “I love you. There is a shooting.” He made his way to the school, where he found Devan walking in a crowd. He waded in and hugged her.

Julie Kellogg was driving by Arapahoe High School when she saw police rush to the campus. Kellogg said she frantically began calling and texting her children at the school but did not hear back. She said she knows they are OK now but the situation was horrifying.

“The shooter started off in the front office and last I heard he was in the library,” Kellogg said, describing what she heard from employees and students who were evacuating the campus. “It was actually the most frightening thing I’ve ever been through.

“I would have never expected my reaction to be what it was,” Kellogg said. “I immediately went into panic, broke down. I didn’t know what to think and I didn’t even know what happened but it I knew it was bad. You could tell it was bad.”

Several hundred parents gathered at Shepherd of the Hills church — stretching tall, leaning this way and that, crying, praying, trying to find their children.

And at the corner of University and Dry Creek, Christina Long stood in tears, staring at the school.

“This doesn’t happen at this high school,” she said. “My baby is in there.”

Long said she didn’t want to text her 16-year-old son, Dylan, a junior at Arapahoe.

“He’ll call me when he’s safe,” she said. “I’m going to let him hide.”

A few minutes later, she got a phone call and broke down in tears. She leaned over and simply said “Oh Baby. OK. OK. He’s OK.”

Then she sprinted across the street.

Kieran Nicholson: 303-954-1822, knicholson@denverpost.com or twitter.com/kierannicholson

Denver Post reporters Lynn Bartels, Kevin Simpson, Zahira Torres, John Ingold, Carlos Illescas and Jeremy P. Meyer contributed to this report.