LOS ANGELES — Los Angeles police and state authorities are investigating a deadly accident involving an LAPD officer who apparently drowned during a training exercise at a backyard pool in Huntington Beach, with officials looking into potential violations of safety protocols related to diving at a private residence rather than a department facility.

The officer who died, Thomas Denton, served in the department’s Metropolitan Division as a diving officer. Denton may have been in the pool testing out a device known as a closed-circuit rebreather, according to two law enforcement sources who asked not to be named in order to discuss an ongoing investigation.

Such devices allow a diver to stay underwater longer by filtering their exhaled breath to remove carbon dioxide. The technology is favored by the U.S. Navy SEALs because it allows their personnel to go undetected by preventing telltale air bubbles from surfacing.

Within the LAPD, rebreathers have historically only been used by the SWAT team’s tactical diver unit, which is tasked with clandestine tasks such as sweeping boats for explosives and attaching trackers to their hulls. The Underwater Dive Unit — of which Denton was a member — typically performs searches for evidence or missing persons.

Experts say the devices can be potentially dangerous for untrained divers, who may not pick up on the signs of oxygen depletion, a condition known as hypoxia.

Just as dangerous is the opposite issue: hyperoxia.

“Because of the way it was being operated or because of some failure inside the rebreather, it puts too much oxygen in the mix,” said David McMurdie, co-founder of the California Recovery Divers and a former chief of the Alameda County Sheriff’s Department’s dive team. “And because it causes this sort of neurological response, the first signal you have that something’s gone wrong is you go into a seizure and you die because you’re underwater.”

This, he said, is why even experienced divers are required to go through a strict checklist of safety checks — a step that some tend to overlook “because it’s a pain in the neck.”

The incident occurred Nov. 5 at a pool in the backyard of a residence in the 17000 block of Crown Circle. Huntington Beach firefighters were mobilized around 11 a.m. for what a 911 dispatcher described as the “drowning” of a 46-year-old male “dive training in the pool.” Denton was taken to an area hospital, where he later died, officials said.

LAPD Chief Jim McDonnell acknowledged Denton during his public remarks at Tuesday’s Police Commission meeting, saying the department was focused on supporting the officer’s grieving family and colleagues.

“His passing has deeply impacted our department,” McDonnell said.

The department responded to a list of questions from The Times with a brief statement that said it was “focused on Officer Denton’s loved ones and the terrible loss they have suffered.”

“The Department is in the process of conducting a comprehensive investigation and will make the public aware of its findings at the appropriate time,” LAPD spokesperson Lt. Elpidio Orozco said in the statement.

The circumstances around Denton’s death are also being reviewed by the California Division of Occupational Safety and Health (Cal/OSHA), which routinely investigates workplace accidents. A spokesperson for the agency said it had no comment, but that in general it has up to six months to investigate an incident and determine whether there were any safety violations.

A Orange County Sheriff’s Department spokesperson said that the coroner’s office there has Denton’s body and plans to perform a full autopsy, the results of which may not be released for six to eight months.

The incident has raised questions about whether the officers present followed department protocols. It’s not uncommon for officers to occasionally train at non-department buildings and facilities, but they are required to obtain prior approval from the entire chain of command, up to their senior captain. They are also required to have an approved injury prevention plan.

Public records show that one of the homes on that street where the incident occurred is owned by the relative of a man with the same name as a Metro officer who worked with Denton.

Denton was a member of the Undercover Dive Unit, which has been around for decades. According to a department Facebook post, the unit was created in 1957 and as recently as few years ago had 25 members who were “responsible for handling and processing underwater accident and crime scenes.”

In recent months, the unit was called on to dredge the pool of a suspect in federal Armenian organized crime investigation.

After Denton’s death, tributes began to pour in from past and present LAPD officers and elected officials. California Gov. Gavin Newsom said in a statement that he had ordered flags at the State Capitol to be flown at half-staff to honor Denton’s “lasting legacy to the people of Los Angeles.”

“Jennifer and I deeply mourn his loss and our thoughts are with his family, friends, and colleagues,” Newsom’s statement read. “May he be remembered for his dedication and honorable service.”

Denton is survived by his wife and two children, the governor’s office said.

Gregory Hoskins, a former LAPD sergeant who supervised Denton at Metro Division, called him an “outstanding” officer who had earned the respect of colleagues for his dependability and work ethic.

“He was a pleasure to supervise, truly, and I’m not just saying that because he’s gone,” Hoskins said.

On social media, videos and photos captured a procession to the Fountain Valley hospital where he was taken.

According to a 2008 statement, Denton was involved in a fatal shooting that year when a suspect lunged at him and another officer with a knife while they were working a narcotics case. Both officers shot the suspect, 56-year-old Jesse Moore, and killed him. Denton had been hired by the department about two years earlier.

He was also among a group of Metro officers who were accused in 2020 of deliberately misidentifying people as gang members. The LAPD presented the case against Denton to the the L.A. County District Attorney’s office, but prosecutors wrote in a memo about the case that there was not enough evidence to charge him.

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