LOS ANGELES (AP) — A 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 Thursday that finds both officers acted lawfully when they fired their weapons.

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.

Ventura County Sheriff’s Sgt. Ron Helus had returned Long's fire from inside the dark and smoky bar and was moving back out the front door when his feet got tangled in a rope and he tumbled to the ground, according to an investigation by the county district attorney's office.

California Highway Patrol Officer Todd Barrett had already exited and was firing at Long, who was still inside, when one of the officer's rounds hit Helus, who had gotten up again and was just outside the door. Helus was shot five times by Long. 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.

Barrett “honestly and reasonably believed” that he and Helus, along with another responding officer, highway patrol Officer Lidia Espinoza, faced imminent threat of death or great bodily injury when they responded to reports of gunfire at around 11:20 p.m. that Wednesday, the report said.

Prosecutors concluded Barrett's and Helus' use of deadly force against Long was justified as a lawful act of self-defense and defense of others.

In addition, Barrett's accidental shooting of Helus was justified and not criminal, the report found.

The report includes edited video from security footage and officer vest-worn cameras. Long is seen waiting in a back office of the bar and watching on video monitors as Helus, Barrett and Espinoza arrive.

“In an extraordinary act of heroism due to the active threat Long posed, Sergeant Helus and Officer Barrett entered the Borderline without knowledge of where Long was located, which afforded Long a substantial tactical advantage,” prosecutors wrote.

At one point, grainy footage shows Long from behind leaning over a counter at the bar's entrance and shooting at the officers. Helus is hit, but he still manages to return fire while retreating, prosecutors said. He was pronounced dead at a hospital.

“It is a testament to his training, character, and valor that even though he had been shot in the heart, he found the strength and determination to continue to protect others and fight to the end,” prosecutors wrote.

Long is also seen lighting and throwing one of several smoke grenades that he tossed into the club to obstruct what employees and patrons could see before he opened fire. He used a flashlight with a laser sight attached to his .45-caliber semi-automatic pistol as he shot.

Long started shooting at patrons and employees within seconds of arriving. Panicked revelers ran for the doors, dived under tables and piled on top of each other in an effort to dodge the gunfire. Others ran for their lives through other exits or broke through windows and jumped out, injuring themselves in the flight.

According to the report, Long fired 61 rounds over about 20 minutes before killing himself. He had 129 rounds remaining.

The district attorney's investigation is the first official report to be released on the shooting. Still pending are the sheriff’s office review of the law enforcement response and homicide detectives’ in-depth investigation of the incident.

Helus, a 29-year veteran of the department, was close to retirement when he was killed. He is survived by a wife and son.