DENVER (KDVR) More details are emerging about how a 16-year-old student carried out a shooting last Wednesday at Evergreen High School, leaving two other students with critical injuries and killing himself.

The Jefferson County Sheriff’s Office said Tuesday that the first deputy was dispatched to the school at 12:24 p.m., “shortly after calls began to flood 911.” The agency said deputies arrived at the high school within two and a half minutes, and that during that time, the 16-year-old shooter had fired multiple rounds inside of Evergreen High School and injured a student.

“While deputies responded quickly, the actions taken by teachers, staff, and students inside the school undoubtedly saved lives,” the sheriff’s office said. “They relied on the safety protocols and lockdown procedures they have practiced, and those immediate actions made a critical difference.”

A part-time school resource officer had been at the school earlier in the day, but was called away for a nearby crash response. The Jefferson County Sheriff’s Office said the deputy followed protocol, and also said that a full-time officer will be assigned to the school when students return to campus for classes.

The agency said that Desmond Holly, the shooter, then left the high school building, crossed the football field and shot a second student at the corner of South Olive Road and Buffalo Park Road, according to the sheriff’s office.

As of Tuesday, one student at Saint Anthony’s Hospital remained in critical but stable condition, and the student who had been transferred to Children’s Hospital Colorado was still in serious condition.

Then, deputies located and confronted Holly, “who then took his own life.” The agency said that there was one shooter in the incident, and approximately 20 rounds were fired. Officials have said they do not know how Holly got hold of the gun.

The Jefferson County Sheriff’s Office said that most school shootings end in minutes, with federal studies showing that active shooter events end in five minutes or less. That timeframe for school shootings is closer to 10-12 minutes, according to the sheriff’s office, and the incident at Evergreen High lasted nine minutes.

“Our hearts are heavy for the two students who were wounded and for the trauma this community will carry long after today,” the sheriff’s office said. “At the same time, we are deeply grateful for the courage and calm shown by Evergreen High School students, teachers, administrators, and staff. Their training, quick thinking, and resilience helped prevent an even greater tragedy.”

The sheriff’s office investigation into the shooting is ongoing. Last week, the agency said that Holly had brought “quite a bit” of ammunition to the school along with a revolver.

Holly’s actions inside the school were tracked through security cameras. Some students were at lunch at the time of the shooting, while others were in the hallways and outside the school building. When gunfire started, some students ran away, while others found areas to lock down.

Law enforcement says that Holly tried to access areas that were locked down but could not enter.