SKIPPACK TOWNSHIP, Pa. (AP) — An explosive device unexpectedly detonated during a training exercise Thursday at a state prison in Pennsylvania, authorities said, leaving five bomb technicians injured.

The accident at the SCI Phoenix prison in Skippack Township occurred shortly before 10 a.m. The training session was being conducted by the FBI’s Philadelphia field office and those in attendance included representatives from several state, county and local law enforcement agencies.

The injured bomb technicians included one who serves with the FBI, one from the Pennsylvania state police and three who serve with the Montgomery County Sheriff’s Office, authorities said. The five were being treated at hospitals for undisclosed injuries that were not considered life-threatening, but their names and further details on their injuries have not been disclosed.

The cause of the accident remains under investigation. The FBI and the Pennsylvania state police are leading the investigation.

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