A Ventura County Sheriff's sergeant accidentally shot by a fellow police officer as they responded to a mass shooting at a California bar had tripped and fallen during the chaos and was struck by the fatal bullet when he stood up and tried to retreat. according to a prosecutors' report released Thursday.

The report by the Ventura County District Attorney's office provides the most detailed timeline of events yet of the massacre that unfolded Nov. 7, 2018, when Ian David Long attacked country-music lovers at the Borderline Bar and Grill in the Los Angeles suburb of Thousand Oaks, killing 11, CBS News reports.

The report includes video that shows Ventura County Sheriff's Sgt. Ron Helus returning fire against the active shooter even after being shot in the heart.

California Highway Patrol Officer Todd Barrett had already exited and was firing at Long, who was still inside, when one of the his rounds hit Helus, who had gotten up after falling and was just outside the door. Helus was shot five times by the active shooter. But the medical examiner determined shortly after the incident that the sergeant was killed by a sixth bullet fired by Barrett that pierced his heart.

Long, a Marine veteran, took a tactical position and lay in wait for the officers, investigators said. The 28-year-old gunman, who wasn't hit by either officer's gunfire, fatally shot himself after the firefight.