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CENTENNIAL, Colo. - Students and staff inside Arapahoe High School say they heard the sound of a shotgun when a student opened fire Friday.
"All of a sudden we heard this, 'bang, bang' really fast and then this girl came running down the stairs screaming, 'Help me, help me. There's a shooter, there's a shooter,' and she just had blood all over her shirt," said student Courtney Leytoldt.
"It sounded like everything was happening just outside of our room," student Ethan Bilek told 7NEWS by phone. "It was really scary."
Bilek said his teacher quickly turned out the lights and put the classroom on lockdown.
"We were shaking, some of us were crying," Bilek said. "We were texting family and loved ones just to let them know we were okay."
"It was terrifying because we didn’t know what was happening but we knew it was real," sophomore Sarah Jenkins said.
"We knew it wasn't a drill because of the way our teachers handled it," said Afton Phelps, who was in gym class at the time of the shooting. "They were pretty serious."
Custodian Fabian Llerenas told 7NEWS that he saw a student dressed in military or tactical gear running through the school.
"It just looked weird," Llerenas said. "He went in and I heard two pops. That's when I knew. I said, 'They are shooting in the school.'"
Llerenas called for help and said he took the threatened teacher with him to his truck.
Arapahoe County Sheriff Grayson Robinson says the gunman entered the school, made his presence known and asked for a teacher by name. That teacher left the building in what Robinson described as a wise "tactical decision."
Robinson initially said a student confronted the gunman, but later said the student may have simply been in the area and was shot. That student is in critical condition at Littleton Hospital.
"We were just in the library doing some work and we heard a loud bang. I thought it might have been someone dropping something, but then I heard 2 more," senior Max Minne told CALL7 Investigator Keli Rabon. "I knew it was a gun for sure."
Minne said he started to approach the front door of the library to see what was happening.
"The shooter just ripped through the door, started yelling 'Where's Murphy?' -- That's the main librarian of the school. I saw him bust through the door of the library with a gun, so I just took off and started yelling, 'Everyone get out, everyone get out.'"
Minne said he exited the library through a back door with other students. It led them through a classroom and back into the hallway near the west entrance.
The hallway, Minne said, was filled with smoke.
"There was a girl on the ground and she was bleeding out of her stomach -- at least that's what I thought it was. I was sprinting so fast that I didn't really get a look at her. She was on her knees," he said.
"It was just tunnel vision; I was just trying to get out as quick as I can."
Most of the students in the building were locked down inside for more than 90 minutes before being escorted out to the athletic fields and then to one of two locations where they were reunited with their parents.
"Sometimes we would hear yelling or dogs barking," Phelps said. "We knew it was the swat team but it was still scary."
Outside, concerned parents clogged the area around the school.
"We talked to her, so we know she's okay. I was really happy," Lorainne Rowley said of her daughter, an Arapahoe High School student. "The best thing ever [is] to know that she's okay and all of her friends are okay."
Another mother, Gelina Mock, said, "I was very very proud of our schools and the procedures that they took action on and put in place. They have done a wonderful job."
Coverage: Arapahoe High School Shooting
Students returned to classes at Arapahoe High School on Tuesday.
Teachers and staff went back to work at Arapahoe High School on Monday. Students will return to classes on Tuesday.
An online petition drive is asking Littleton Public Schools to give Arapahoe High School shooting victim Claire Davis a diploma at graduation in May.
When an armed student went into Arapahoe High School on December 13, many people sprung into action.
Claire Davis' father asked people to forgive the fellow student who shot and killed his 17-year-old daughter at Arapahoe High School.
The Arapahoe High School shooter entered the school through a door that should have been closed and locked, but was left ajar, Arapahoe County Sheriff Grayson Robinson said.
Gov. John Hickenlooper orders flags lowered to half-staff Wednesday in honor of Claire Davis, the student slain in the Arapahoe High School shooting.
Tickets to the celebration of Claire Davis' life are now available for students and staff at Arapahoe High School.
Claire Davis will be celebrated next week with a memorial at the National Western Stock Show and Complex.