Authorities say as many as five law enforcement officers were injured when an explosive device unexpectedly detonated during a training exercise at a Montgomery County jail.

Five law enforcement officers were injured during a bomb squad training exercise at a state prison in Skippack Township, about 30 miles northwest of Philadelphia, on Thursday morning, state police said.

Three Montgomery County Sheriff’s Office bomb technicians were injured, as well as a Pennsylvania State Police officer and an FBI agent in Philadelphia, the FBI said in a statement late Thursday.

Among those injured is a 14-year veteran of the sheriff’s office who was airlifted to Penn Presbyterian Medical Center. A 13-year veteran and a 12-year veteran at the sheriff’s office were also injured. They were treated at Paoli Hospital, county officials said.

FBI Philadelphia said a training device unexpectedly detonated while the agency was “conducting explosive response training at SCI Phoenix for law enforcement partners.”

State police tweeted Thursday afternoon that their bomb technician suffered non-life-threatening injuries. The conditions of the others are not known at this time.

It is not clear what led to the incident. The FBI and state police are investigating the incident.

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