THOUSAND OAKS (CBSLA) – The Ventura County Sheriff’s sergeant killed during the mass shooting at the Borderline Bar & Grill last month in Thousand Oaks was struck by friendly fire during a shootout with the gunman, authorities said Friday.

Sgt. Ron Helus, 54, was struck by five rounds from the suspect’s gun and a sixth round from the rifle of a California Highway Patrol officer. It was that sixth round which struck Helus in the chest and heart and ultimately killed him, authorities reported Friday morning.

“Today I’m deeply saddened to inform you that Sgt. Helus was also struck by a sixth bullet, which we now know through forensic analysis from the FBI’s crime laboratory, was fired from the CHP officer’s rifle,” Sheriff Bill Ayub said at a news conference. “Tragically, that bullet struck vital organs, and was fatal.”

Helus and the unnamed male CHP officer were the first law enforcement officers to respond to the shooting. They entered the bar together.

“When Ventura County Sheriff’s Sgt. Ron Helus and a CHP officer entered the bar, the suspect attacked almost immediately, firing multiple rounds at both men,” Ayub said. “Sgt. Helus was struck five times by gunfire from the suspect. Both men quickly retreated and fired their rifles at the suspect.”

It was at that point that the CHP officer shot Helus once.

“Whether this was accidental, or a missighting, or perhaps a mistaken identity is something we’re still investigating,” Ayub said.

CHP Coastal Division Chief L.D. Maples described the CHP officer, a nine-year veteran, as a “consummate professional” with a “well-trained military background.” Maples said the officer didn’t realize he had killed Helus until he was informed Thursday of the results of the FBI investigation.

“I delivered the message to him yesterday, he had no clue it was coming,” Maples said. “It was a surprise to all of us. He was devastated.”

The unidentified CHP officer was on voluntary leave, according to CBS2’s Randy Paige.

Helus, a 29-year veteran of the department, was one of 12 people killed when 28-year-old Ian David Long, a Marine Corps. veteran, opened fire on hundreds of people at the Borderline bar, located at 99 Rolling Oaks Drive, late on the night of Nov. 7. Long is believed to have died from a self-inflicted gunshot wound.

LATEST COVERAGE: The Borderline Massacre

Ventura County Medical Examiner Dr. Christopher Young noted that Helus could have potentially survived the five initial rounds.

“These bullets caused serious injuries, but potentially survivable injuries,” Young said.

Young explained that the five bullets from Long’s gun were discovered during the autopsy. However, the sixth bullet was found during emergency surgery on the night of the shooting, after Helus was rushed to Los Robles Hospital, where he died at around 2 a.m. on Dec. 8.

“This (bullet) struck Sgt. Helus in the chest, and struck his heart, ultimately,” Young said. “This is sad news, and a tragedy, but ultimately this was the most severe injury sustained.”

Young said forensic pathologists had suspected it was possible the CHP officer’s bullet may have been the one that killed Helus, but had to wait for the FBI’s investigation to confirm it.

“We were aware of the injuries sustained by Sgt. Helus, but we needed confirmation from ballistics, and it wasn’t certain that it was friendly fire,” Young said.

Helus was killed despite wearing a protective vest.

“Two of the rounds fired struck the protective portion of the vest, but only near the very edge of the vest, so, in my mind, decreasing the protection provided,” Young said.

Long was armed with just one gun that night – a Glock .45-caliber handgun. However, he fired more than 50 rounds and had seven high-capacity, 30-round clips on him – a total of 230 bullets. He also had a flashlight with a laser sight.

“It is also important to note that the news I have shared with you today, in no way diminishes the heroic actions both men exhibited at the Borderline,” Ayub said.

“The burden lies solely with him (Long), not those who tried to save lives, those who tried to escape, and certainly not with those who died while simply trying to enjoy an evening with friends,” Ayub added.

At a previous news conference late last month, Young explained that most of the victims were shot at close range, and that all of them had died quickly and could not have survived their wounds.

The victims in the shooting included Alaina Housely, 18, a Pepperdine University freshman and niece of actress Tamera Mowry; 22-year-old Cody Coffman, who was getting ready to join the Army; Justin Meek, 23, a recent Cal Lutheran graduate; Sean Adler, the owner of Rivalry Roasters in Simi Valley; 21-year-old Noel Sparks, a Moorpark College student; Kristina Morisette, 20, who had been working as the cashier at the front door of the bar that night; Jake Dunham, 21, of Newbury Park; his friend 21-year-old Blake Dingman; Telemachus Orfanos, 27, who had just survived the Las Vegas mass shooting a year ago; and 33-year-old Daniel Manrique, a Marine Corps veteran who was helping veterans adjust to civilian life.