Investigators in Colorado are looking at whether revenge motivated a teenage gunman to enter his suburban Denver high school armed with a shotgun looking for a specific teacher.

LATEST:

 

Parents wait at Shepherd of the Hills Church to be reunited with their children after a school shooting in Centennial, Colorado (Chris Schneider/Getty Images)

11:20 p.m. - The gunman has been identified by Arapahoe County Sheriff Grayson Robinson as Karl Halverson Pierson, 18.The sheriff did not elaborate on any possible motive except to say Pierson had a "confrontation or disagreement" with the teacher. He didn't know if the injured girl was intentionally shot.

Pierson made no attempt to hide his weapon after entering the school from a parking lot and asking for the teacher by name, Robinson said. 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," the sheriff said. "That was a very wise tactical decision."

The wounded student, a 15-year-old girl, underwent surgery and was in critical condition. Authorities originally said a second student was wounded, but Robinson said that the other girl taken to a hospital was covered in blood from the first student and wasn't injured.

6:00 p.m. - Robinson said the student is believed to have acted alone. Authorities also found a possible Molotov cocktail at the scene and are examining the device.

4:33 p.m. - A janitor tells 7 News Denver the student shooter was dressed in tactical gear running through the school. The janitor said he heard two shots and called 911.


Track at Arapahoe High School in Colorado (CBS 4 Denver)

At a press conference Arapahoe County Sheriff Grayson Robinson  confirmed that the shooter was a student at Arapahoe High School and took his own life. He would not identify the student. The shooter was found while the school was on lockdown 15 minutes after police arrived

 

Robinson said the armed student entered the school looking for a specific teacher who was notified left the school. A female student confronted the shooter and was critically wounded. another student is also seriously injured but not necessarily by a gunshot. A third student was taken to a hospital with minor injuries.

A police SWAT team is on the scene at the school attended by 2,000 students.

"I was scared and shaking," one told the Denver Post. "She said she heard, "bang, bang, bang" and by the third shot she was on the ground."

Students inside the school were being told to not use their cell phones while other students are leaving the school with their hands over the heads on the school's track.

Students are being taken to Shepherd of the Hills Church in Centennial.

Arapnahoe High School is in a community on Denver's southside. The school is 8 miles from Columbine High School which was the scene of a shooting in 1999. The shooting occurs one year to the day of the shooting in Sandy Hook, Connecticut.

MORE COVERAGE:

The Associated Press contributed to this report.