Authorities wanted this mock school massacre to feel as realistic as possible.
Two fake shooters in dark sunglasses begin stalking students on a crowded campus, setting off a chaotic scene of screaming children running for cover amid cracks of gunfire and bomb smoke.
Some student actors, covered in blood-red paint, were heard crying for help as others, as young as 12, locked themselves inside barricaded classrooms.
The shooters were even seen picking their way through the bodies of students playing dead, nudging some with the end of their boot to see if they needed finishing off.
"I felt really terrified," Kianna Whitten, one of 20 teenage actors who took part in the exercise, told NBC. "I've never felt that scared."
Patrol cars, SWAT vehicles and ambulances eventually swarmed onto the campus to rescue the students, as a videographer captured all the action on camera.
The simulated massacre was played out at a school in the northern Californian town of Modesto on Sunday, so authorities can better prepare for such emergencies in the future.
The training video that will be produced from the exercise is to be distributed next year to school administrators and law enforcement agencies throughout California, authorities said.
"We need to actually know how our guys at the Sheriff's Office are going to respond to an actual emergency and the best way to do that is through live simulated training, making it as real as we can," Sergeant Anthony Bejaran from Stanislaus County Sheriff's Office said.
Sunday's exercise was designed in the aftermath of the Sandy Hook Elementary School shooting in Connecticut two months ago, in which a gunman fatally shot 20 children and six adult staff members.