By Rebekah Riess, Josh Campbell, Emma Tucker

(CNN) — An explosion at a Los Angeles County Sheriff’s Department training facility killed three veteran members assigned to the agency’s arson and explosives detail Friday morning – a tragedy that marks the “largest loss of life” at the department since 1857, according to Sheriff Robert Luna.

The explosion is believed to have been an accident. Federal and local investigators began working to determine what led to the explosion after the scene was rendered safe by the Los Angeles Police Department’s bomb squad just after 11 a.m. local time, Luna said at a news conference.

The LASD’s bomb squad responded to a call in the county on Thursday involving an explosive device, which was transported back to the training facility located at the agency’s headquarters, law enforcement sources briefed on the investigation told CNN’s chief law enforcement and intelligence analyst John Miller.

While the investigation still has to determine whether that device was a piece of military ordnance or an improvised explosive device, the sources said the plan was to perform a render-safe procedure on Friday morning.

“That means they either countercharge it and blow it up in an open area where they can do that, or they perform a more intricate procedure where they take it apart and render it safe from detonating,” said Miller.

At some point while moving that device Friday morning, possibly from another vehicle or to where they were going to render it safe, the device functioned, according to Miller, citing law enforcement sources.

The three sworn members had collectively served for 74 years and authorities are not releasing their names at this time, the sheriff said. Luna added he’s met with several family members of the deputies, but some have not yet been notified.

“The individuals who work our arson explosives detail, they have years of training,” Luna said, adding the members respond to an average of 1,100 calls per year, which include “dealing with some very dangerous situations or items.”

“These aren’t people who don’t do this very often. They are fantastic experts, and unfortunately, I lost three of them today,” the sheriff continued.

The incident happened shortly after 7:30 a.m. local time at the Biscailuz Center Training Academy, which houses the sheriff’s department’s special enforcement units and bomb squad, according to a release from the LASD. The facility is located about 6 miles east of downtown LA.

“At this time, we do not know the cause of the explosion … I can tell you, there is no threat to this community. This is an isolated incident,” Luna said.

US Attorney General Pam Bondi and the LASD said earlier Friday the three members died in a “critical workplace incident.”

“I just spoke to (US Attorney for the Central District of California Bill Essayli) about what appears to be a horrific incident that killed at least three at a law enforcement training facility in Los Angeles,” Bondi said in a post on X. “Our federal agents are at the scene and we are working to learn more. Please pray for the families of the sheriff’s deputies killed.”

“Sheriff’s homicide detectives are on scene,” the LASD said.

The FBI is responding to assist with the incident, according to the FBI’s Los Angeles field office. Agents with the Bureau of Alcohol, Tobacco, Firearms and Explosives are also on scene, according to Bondi. LASD homicide detectives are also on scene investigating the deaths, Luna said.

Los Angeles Fire Department arson investigators and members of the Los Angeles Police Department’s bomb squad are assisting at the training facility, LA Mayor Karen Bass said in a post on X.

The work of the bomb squad was complicated by the fact officers had to ensure no other explosive material that could be stored there was vulnerable to detonation, the senior law enforcement source told CNN.

Meanwhile, Los Angeles officials are expressing heartbreak over the tragedy and rallying around the sheriff’s department in solidarity, ready to offer assistance as needed.

The LAPD’s bomb squad, SWAT and K-9 teams are actively responding to calls on behalf of the sheriff’s department so the agency can “focus fully on supporting their personnel and conducting this critical investigation,” Police Chief Jim McDonnell said.

Los Angeles County Board of Supervisors Chair Kathryn Barger said she is “closely tracking the situation as we learn more about what occurred.”

Barger continued: “My heart is heavy, and my thoughts are with the brave men and women of the Sheriff’s Department during this difficult time. We stand with them and their families as they navigate the hours and days ahead.”

This story has been updated with additional information.

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