In 1999, 13 people were killed in the Columbine High School massacre - about 25 miles southwest of Friday shooting.
Ben Fernandez and his girlfriend were in the theater next door when alarms went off - and first thought it was a false alarm.
"The alarms went off and there was just a voice over the intercom saying, 'You have to exit the theater, there's an emergency,' and we thought it was just kind of a false alarm so we just kind of stood there trying to watch the movie still," Fernandez told "CBS This Morning."
"And then when we saw the police, you know, they weren't even really ushering people out, like calmly. They were pretty much yelling at everybody, screaming at them that they need to get out, telling people to just sprint out of the theater, and that's when we realized something bad was happening."
After going outside Fernandez said he saw several armed police officers.
"We walked to the left in front of the theater and we saw a young girl, maybe like 12, 13, just laying on the concrete. From what I saw, she had two bullet wounds in her leg and she had just blood all over her stomach. Her friends were all just yelling, crying. There were people just saying, 'My friend just got shot.' We were just all pretty much in shock," Fernandez said.
He described people who had gathered in different crowds, some wounded. "We saw police officers helping out a man that was just covered in blood. We didn't know what happened to him - he was just completely out of it and covered in blood. Like i said. It was just really scary."
Then, Fernandez said, a large crowd started sprinting away from the theater. "We weren't sure, you know, if people were going to come out like shooting guns or what was going on. So we all just kind of took off running towards the car. It was just pretty much like panic and pretty much chaos like at first . . . I feel shaky even talking about it right now. It was just a lot of crazy, crazy stuff going on that I've never seen before in my life."
Fernandez said he did not see the shooter. "As we were leaving, the police were questioning everybody coming out, trying to see what was going on, if anybody saw the shooter up front."
Witnesses and people who were evacuated from apartments nearby were being taken to Gateway High School for interviews and also to reunite with friends and family.
At least 24 people were being treated at Denver area hospitals.
The youngest victim reported was a 3-month-old, who is reportedly doing fine at University Hospital, where nine others are in critical condition.
Two people in critical condition were rushed to nearby Swedish Medical Center, spokeswoman Nicole Williams said.