Some of the first responders weren't even on duty at the time, but they ran toward the gunfire and, for some, the memories are still painfully vivid.
It had started as just another day for Los Angeles County Sheriff's detective Daniel Finn. He dropped off his girlfriend's son at the school and started to drive away when he saw the commotion.
"(I noticed that) kids were running in all directions. They were running north through the neighborhood, they were running east toward Bouquet Canyon," Finn said. "I was off duty and so I was in plain clothes. I had my pistol with me, I started working my way down to the center of the campus."
He says that's when an off-duty Inglewood officer joined him. In the school's quad area, they saw several students on the ground.
"Once I saw the victims, I realized that we did have a shooter on campus. There is no way to really prepare when you're in the middle of it, and when you've got students that are victims of gunshots," Finn said.
First responders had to determine whether or not there was still an active shooter on the scene before they could fully begin the rescue process.
"As it turned out, the shooter was one of the victims at our location. But we didn't know that immediately and, again, we didn't know if they were multiple shooters," he added.
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"What it does illustrate though is the fact that the first responders that arrived we're off-duty deputies," L.A. County Sheriff Alex Villanueva said.
Villanueva said that despite efforts to remove law enforcement from campuses, the shooting showed that they are a needed presence.
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"When we have school resource deputies or officers present on campus, that's someone who can respond immediately," he added.
A year later, as he thinks about that day, it is still very difficult for Detective Finn to talk about what happened. He said he did his job, but it was the teachers and the children that did all the right things.
"They were the real heroes that day, the way they reacted, the kids did everything, everything absolutely right. They had had a drill, I believe, a week or two prior. They did everything right, the teachers did everything right. They're the ones that truly saved lives," Finn said.