Three Los Angeles County sheriff’s deputies assigned to the arson explosives detail were killed in an explosion at a law enforcement training facility.

It was not immediately clear what caused the blast at the Los Angeles County Sheriff’s Department’s Biscailuz Training Academy center. No one else was injured, officials said.

Three deputies were killed Friday in an explosion at the L.A. County Sheriff’s Department’s Biscailuz Center Training Academy in East L.A., the deadliest incident for the agency in more than 160 years.