An explosion at a Los Angeles law enforcement training facility early Friday that killed three deputies was being investigated as a possible training accident, officials said.

Los Angeles County Sheriff Robert Luna said the deputies were members of the arson and explosives unit, a team that goes through in-depth training and responds to more than 1,000 calls a year.

“They have years of training,” he said. “They are fantastic experts and, unfortunately, I lost three of them today.”

The explosion was reported about 7:30 a.m. at the Biscailuz Training Facility, Los Angeles County Sheriff’s Department spokesperson Nicole Nishida said. It was not immediately known what caused the explosion or what the deputies were doing at the time.

Luna said it took more than four hours to render the scene safe and the deaths are being investigated by the department’s homicide detectives, who are being assisted by the FBI and the Bureau of Alcohol, Tobacco, Firearms and Explosives. No one else was injured in the explosion, he said.

An early line of investigation was looking at a possible training accident, according to a law enforcement official briefed on the matter who was not authorized to discuss it and spoke to The AP on condition of anonymity.

In a post on X, Attorney General Pam Bondi said the explosion “appears to be a horrific incident” and federal agents are at the scene to learn more.

“Please pray for the families of the sheriff’s deputies killed,” Bondi wrote.

Altogether, the three deputies had served in the department for 74 years, Luna said. He said the deaths marked the department’s worst loss of life in a single incident since 1857 and noted that he couldn’t release the deputies’ names because he had yet to speak to one of the families.

“I have met with two of three families thus far. Those were extremely challenging conversations,” Luna said, his voice breaking.

Arson investigators from the Los Angeles Fire Department and members of the Los Angeles Police Department bomb squad were also assisting the investigation at the training facility, Los Angeles Mayor Karen Bass said in a post on X.

California Gov. Gavin Newsom’s office said he’s been briefed and that the Governor’s Office of Emergency Services is in contact with the Sheriff’s Department and closely monitoring the situation. He later posted on X that members of the State Fire Marshal were helping with the investigation at the request of the ATF.

Aerial footage from KABC-TV shows the explosion happened in a parking lot filled with sheriff patrol cars and box trucks. Three covered bodies could be seen near a truck with a ramp attached to a side door. A sheriff’s patrol cruiser parked nearby had its rearview mirror shattered by the blast.