Three members of the arson and explosives unit of the Los Angeles County Sheriff’s Department were killed Friday in a blast at the department’s training facility.

It was not immediately clear what they were doing when the explosion happened in a parking lot at the Biscailuz Training Facility or what caused it. However an early line of investigation is examining whether it may have been a training accident, according to a law enforcement official briefed on the matter who was not authorized to discuss it publicly and spoke on condition of anonymity.