Three people were killed Friday in an explosion at the arson and explosives unit at Biscailuz Center, the Los Angeles County Sheriff’s Department’s main training academy in East L.A., authorities said. One other person was injured.

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