The Orange County Sheriff's Department said one person suffered a leg injury and will need surgery, while two others had superficial wounds that won't require surgery. The remaining 13 had dizziness and ringing in their ears.
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The incident happened shortly before 1 p.m. at an FBI training facility in Irvine, California.
It was not immediately clear what caused the explosion.
Most of the SWAT members have been released from the hospital, sheriff's Sgt. Frank Gonzalez said.
No FBI personnel were injured, said bureau spokesperson Laura Eimiller.
The facility hosts firearms training and qualifications tests for the FBI and local law enforcement agencies, Eimiller said.
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The FBI and the Sheriff's Department will investigate.
The Associated Press contributed to this report.