FRESNO, Calif. -- Authorities said 11 people were hurt, at least three of them critically, in an explosion Friday in the central California city of Fresno, CBS San Francisco reported.

The explosion happened at the Fresno County Sheriff's gun range at around 2:30 p.m., according to Tony Botti, a spokesman for the department.

Botti said a work crew which included county jail inmates using heavy equipment apparently hit a Pacific Gas & Electric Co. pipe carrying natural gas.

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One of the workers was among the three people in critical condition, Botti said. Two of them were flown out by helicopter for medical treatment. No law enforcement officers were hurt, he said.

Witnesses reported seeing a large fireball shooting about 200 feet into the air, according to Pete Martinez of the Fresno Fire Department. The explosion caused a two-alarm grass fire.

Gas lines are shut down, Fresno Fire continues to monitor atmosphere and patrol the area around grass fire. pic.twitter.com/5eNTgZIKBm — Fresno Fire PIO (@FresnoFire) April 18, 2015

Traffic heading both north and south on Highway 99 in Fresno was halted, the California Highway Patrol said. A nearby rail line was also shut down out of concern that a passing train could spark leaking gas.

The CHP asked drivers to use alternate routes as it was unclear when the highway will reopen.

The fire department and PG&E said the circumstances of the explosion would be investigated.