An army of police officers responded and terrified parents rushed to the scene after a student entered Arapahoe High School in Colorado and shot two students, authorities said. NBC's Tom Costello reports.

Karl Halverson Pierson

A quick-thinking educator may have saved other lives Friday by trying to lure a student brandishing a shotgun away from teenagers at a Colorado high school, where the gunman wounded a fellow student before killing himself, authorities said.

The injured student, a 15-year-old girl, was critically wounded and was in surgery after the gunman shot her at Arapahoe High School in Centennial. Arapahoe County Sheriff Grayson Robinson said initial reports that the student confronted the gunman turned out not to be true.

A second student initially believed to have been wounded turned out only to have been splattered with some of the girl's blood, Robinson said at a news briefing late Friday.

The shooter, identified as Karl Halverson Pierson, 18, a student at the school, appeared to be seeking revenge against a specific school staff member after an earlier confrontation, Robinson said. He said Pierson was looking for the educator by name when the incident began about 12:33 p.m. (2:33 p.m. ET).

The staff member was identified as school librarian and debate team coach Tracy Murphy by NBC affiliate KUSA in Denver. Authorities initially described him as a teacher.

"The teacher exited the school immediately, which was, in my opinion, the most important tactical decision that could have been made," Robinson said. "I believe it was a wise tactical decision to remove himself from the school so maybe the threat went toward him rather than the school."

An army of officers responded, having dramatically changed their standing orders after the deaths of 12 students and a teacher 14 years ago at Columbine High School — just about 10 miles away. Instead of waiting for backup, they rushed in immediately.

They found Pierson in a classroom about 20 minutes after they began their search, Robinson said. He said investigators knew his identity and were talking to his family, even as his body remained inside the school Friday night.

Arapahoe County Sheriff Grayson Robinson identifies the teenager who shot a fellow student at Arapahoe High School before killing himself.

Robinson said two devices similar to Molotov cocktails were also found in the building, one of which created a cloud of smoke and the other of which was disarmed. As darkness fell, SWAT officers with dogs were still at the school.

Meanwhile, police, sheriff's deputies and agents of the U.S. Bureau of Alcohol, Firearms, Tobacco and Explosives barreled into a home about 4 miles from the school. No one appeared to be home, a neighbor said.

"An ATF agent said we should stay in the house," she told NBC station KUSA of Denver. "I didn't ask any questions."

Robinson said late Friday that authorities were investigating four locations: the school, Pierson's car, his mother's home and his father's home. His parents divorced in 2011.

Related: Colorado school gunman Karl Pierson seen as bright and passionate about politics

A janitor first spotted the gunman and alerted authorities. The man, who said he wanted to be identified only as Fabian, said the gunman was running side to side, kind of military style."

"I looked to see if there was a gun," Fabian told KUSA. "It was a shotgun."

"When he went into the library, he said, 'Where is Murphy?'" Fabian said.

Robinson confirmed that the gunman was armed with a shotgun.

Students described a scene of terror when the shooting started.

"I heard three shots, and then the school went into lockdown," Camden Flinders, 14, a freshman, told NBC News. "I saw the blood in the study center and in the gym. We could see blood on the floor. ...

"We went on lockdown for about 20 to 30 minutes. There was big line of police and blood splatters by the gym and the study center," he said.

Parents were being asked to gather at nearby schools and churches, the sheriff's office said. They dropped whatever they were doing to rush to the scene — one was seen in full medical scrubs, as though he had just raced out of a doctor's office, KUSA reported.

An Arapahoe High School student tells NBC station KUSA she texted her parents a message she never wanted to have to send them.

Camden's father, Marcus Flinders, was working at an area Latter-day Saints temple when he got the news from a fellow congregant.

"I broke down. I thought my son was one of the ones who was injured," he said. "I was pretty much crying. It was really traumatic."

'A traumatic situation'

Authorities said that while they had no indication of a second shooter, they proceeded as though there might have been one as a matter of caution. Students were led out of the building with their arms in the air, some of them being patted down by police.

"The kids have been through a traumatic situation," Robinson said. "We need to ensure that anyone we evacuate isn't part of our problem. We're doing the evacuation slowly and deliberately."

President Barack Obama was briefed about the shooting, White House officials said.

Arapahoe High School, with 2,000 students, is an affluent suburb of Denver in the same county where James Eagan Holmes is accused of having killed 12 people in a movie theater in the town of Aurora last year.

Asked whether the shooting might have some connection to Saturday's anniversary of the deadly shootings at Sandy Hook Elementary School in Newtown, Conn., Robinson replied simply, "No."

Tom Costello, Pete Williams, Christopher Essner, Azhar Fateh, John Cheang, Simon Moya-Smith, Geoff Tofield, Liza Torres and Daniella Silva of NBC News contributed to this report.

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