(CNN) -- The only sound Christian Myers heard in his classroom at Arapahoe High School was the constant buzz of vibrating cell phones -- calls from panicked parents trying to reach their children.
The sophomore and his fellow classmates were hunkered down in a corner of a darkened room.
Minutes earlier, they had heard the sound of gunshots.
That was followed by a dire warning from a teacher.
"He just yelled, 'Lockdown!'" Myers, a sophomore, told CNN on Friday. "He goes 'Do what would you do in a lockdown drill. This is not a drill.'''
'It was a shotgun'
It was just after 12:30 p.m. Friday at Arapahoe High School with its student population of more than 2,200 in Centennial, Colorado, when the shooting began.
Some students were on lunch break at the open campus; others were still in classes.
Janitor Fabian Llerenas was on the north side of the building when he saw a student running "side-to-side, kinda military style."
The student was carrying something. Llernas did a double take. "It was a shotgun," he told CNN affiliate KUSA.
"So right away, I got on the radio to alert everyone on the staff," he said. "...And that's when I heard the shots."
'A really loud bang'
One minute, ninth-grader Whitney Riley was getting ready to retrieve her computer from her locker at Arapahoe High School. The next minute, she was running.
"We were having fun and laughing and then, all of a sudden, we heard a really loud bang," the 15-year-old told CNN.
"My teacher asked what it was and then we heard two more and we all just got up and screamed and ran into a sprinkler system room."
Inside the windowless room were five students and two teachers. "We were shaking, we were crying, we were freaking out," Riley said. "I had a girl biting my arm."
They soon heard people yelling, and walkie-talkies crackling, and then they heard police asking someone to drop the gun, put the gun down, and hold his arms up, she said.
She did not hear another gunshot, though the people doing the talking could have moved farther away, she said. Soon, they heard police ordering them out.
Eric Waugh was in his physics class when he and his classmates heard a bang.
Initially, the teacher opened the door, he said. The teacher and students "initially thought it might be an explosion from a chemistry class."
Then, they heard a gunshot. And then another. And then another.
The teacher locked the door and turned off the lights. "He yelled at us to go and get in the corner, and we were in the corner in two seconds," Waugh said.
Later, SWAT officers came to the classroom door.
The students filed out in single file, their arms above their heads, he said.
"We never thought this would happen at our school," Waugh said.
'Are you OK?'
Myers, the sophomore, and his fellow classmates were sitting in the dark when he started to think about his twin brother, Austin.
He knew his brother should be in class. Even so, he was worried about him.
"Are you OK?" Myers texted.
There was no answer.
He texted again: "You doing OK?"
Still, there was no reply.
Myers was overwhelmed with fear. Was his brother one of those who had been shot?
He tried several more times to raise his brother by text message to no avail. Then he sent a text message to a friend, who he knew had a class with his brother. Have you seen my brother? Myers asked.
The answer: "Austin never showed up to class today."
Panicked and in tears, Myers reached out to his father by text message: Dad, Austin is missing.
Moments later, his father responded: Austin stayed home today. He wasn't feeling good this morning.
CNN's Jake Tapper and Anderson Cooper contributed to this report.