IRVINE, Calif. (AP) — An explosion during an indoor training exercise Wednesday sent 16 members of Southern California’s Orange County Sheriff’s Department SWAT team to the hospital, with one person requiring surgery for a leg injury, the department said.

Two others had superficial wounds while the remaining 13 had dizziness and ringing in their ears.

The blast occurred shortly before 1 p.m. at the facility in Irvine, said sheriff’s Sgt. Frank Gonzalez. It was not immediately clear what caused the explosion, but it happened during a training with the sheriff’s bomb squad, he said.

The most severe injury was a non-life-threatening leg wound that will require surgery, Gonzalez said. The two other wounded deputies won’t require surgery, he said.

No FBI personnel were injured, said bureau spokesperson Laura Eimiller.

The facility south of Los Angeles hosts firearms training and qualifications tests for the FBI and local law enforcement agencies, Eimiller said.

The FBI will lead the investigation, Gonzalez said.