Story highlights The cause of a gas line explosion in Fresno, California, is unknown

Two of the injured were undergoing emergency surgery

(CNN) A natural gas line explosion at a law enforcement shooting range in Fresno, California, injured 11 people, including some inmates who were on a work detail there.

Others being treated include a county road worker and two sheriff's deputies, Fresno County Sheriff Margaret Mims said.

The exact cause of Friday's blast is under investigation, Mims said, but it happened at the spot where a county worker was operating a front loader.

The explosion sparked a fire that roared like a jet engine, Mims said.

The operator of the front loader was injured but able to walk to an ambulance, the sheriff said.

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