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(CNN) - Arapahoe High School ninth grader Whitney Riley was in the school's study center, laughing and having a good time with fellow classmates. That's when she heard the first bang.
"My teacher asked what it was, and then we heard two more," said Riley, whose parents gave CNN permission to talk with her. "We all just got up and screamed and ran into a sprinkler system room."
The three shots sounded like they came from a nearby hallway. Riley, four other classmates, and two teachers stayed huddled in the room.
"We were shaking, we were crying, we were freaking out," said Riley.
The group stayed quiet, and then began to hear sounds - people yelling, walkie talkies, and police asking the shooter to drop the gun, and put his hands up. The sounds came closer, and then got farther away, says Riley.
Finally they heard the policeman, calling for everyone to come out, and leave the school.