LOS ANGELES (AP) — A Friday blast at a Los Angeles County Sheriff’s Department training facility killed three members of its arson and explosives unit, marking one of the department’s worst losses of life from a single incident, the sheriff said.

All three were veteran deputies. The department hasn’t said what they were doing at the time of the blast or what caused it.

Los Angeles County Sheriff Robert Luna said the arson and explosives team undergoes in-depth training and responds to more than 1,000 calls a year. The deputies had served 19, 22 and 33 years in the department, Luna said.

“They have years of training,” the sheriff said at a news conference. “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.

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

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, with the assistance of 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 Associated Press on condition of anonymity.