According to the LA Times:

A preliminary investigation by a state utility agency revealed Fresno County workers who hit a gas line, triggering an explosion that injured 14, had not checked for underground utilities before work began, officials said.

A county worker leading a team of jail inmates building a dirt road accidentally struck a gas line about 2:30 p.m. Friday, sparking an explosion that sent flames more than 100 feet into the air and injuring 14 people, two critically.

Witnesses described a scene of destruction in the aftermath, with a crater marking the point of the explosion at the gun range.

Spencer Olson was drilling for water nearby.

“My jaw dropped,” he said. “I saw an enormous fireball.”