When the shooter opened fire at STEM School Highlands Ranch in suburban Denver, an 18-year-old charged him, getting shot himself in the process but giving his classmates time to take cover or bolt.
Nui Giasolli had British literature class with one of the shooters who killed a student and wounded eight others at the school Monday, and she shared with the "Today" show a story of terror and heroism that unfolded once bullets began to fly.
The shooter came into the classroom late, she said. He walked over to a second classroom door and opened it. He walked back toward his desk, then returned to the door and closed it, Nui recalled.
She'd known him for four years, she told "Today." He was always nice to her. She never thought he was capable of what followed, she said.
"The next thing I know he's pulling a gun and he's telling nobody to move," she said.
Her 18-year-old classmate, whom CNN is not naming because he has not been publicly identified, lunged at the shooter and was shot, "giving all of us enough time to get underneath our desks, to get ourselves safe and to run across the room to escape," she said.
Three other students also rushed the shooter, Nui said, describing them as "brave enough to bring him down so that all of us could escape and all of us could be reunited with our families."
Standing alongside her grateful mother, Nyki, the young woman told the morning show, "They were very heroic. I can't thank them enough."
One of the three students she mentioned is senior Brendan Bialy. His father told The New York Times that Brendan was in the classroom when two students walked in and one of them pulled a gun out of a guitar case.
Brendan and two friends tried to tackle the shooter, but one of the boys was shot in the chest, Brad Bialy told the paper. Other students tried to stop the bleeding by applying pressure to his chest, the father told the Times.
The youngsters did exactly what they were supposed to do, according to experts. Where people were once advised to flee or shelter in place, the new mantra for surviving an active shooter situation is "run, hide, fight."
The guidance, sometimes referred to as Civilian Response to Active Shooter Events, began to emerge in 2002 upon the realization that the law enforcement response at Columbine High School, which happened 20 years ago about 7 miles away from Tuesday's shooting, was ineffective.
The shooters at Columbine -- just like Virginia Tech, Fort Hood, Sandy Hook and far too many others -- weren't common criminals, experts realized. They weren't after wallets or purses. They wanted body counts, blood and headlines.
CRASE trainers devised a strategy called "avoid, deny, defend," but the premise is the same as "run, hide, fight." If you can't protect yourself, take out the gunman.
"Avoid" means not only to run but to be vigilant. Know where the exits are and visualize how to get out of a room before something goes away. "Deny" includes hiding, but it also means taking other precautions, such as barricading doors, turning off lights and silencing phones. If hiding is the only option, do it behind something that will stop a bullet.
And when "avoid" and "deny" fail, there's "defend," which means fight -- and not fairly. Do whatever it takes to stop the shooter.
A week ago, across the country at the University of North Carolina at Charlotte, Riley Howell, 21, knew he and his classmates might not survive unless someone took down the man who had just opened fire in a lecture hall.
He charged the gunman, knocking him off his feet, allowing a police officer to rush in, disarm the suspect and take him into custody, police said.
Much like that of the 18-year-old in Highlands Ranch, Howell's heroism came at the ultimate price.
"He took the fight to the assailant. ... Unfortunately, he had to give his life to do so, but he saved lives doing so," Charlotte-Mecklenburg police Chief Kerr Putney said. "You're either going to run, you're going to hide and shield, or you're going to take the fight to the assailant. Having no place to run or hide, he did the last."