FRESNO, Calif. – A large gas pipeline exploded into a wall of fire Friday in Central California, closing both directions of a major highway in the region and injuring at least 15 people, four of them critically, authorities said.

Four patients were being treated at Community Regional Medical Center’s burn and trauma unit, spokeswoman Mary Lisa Russell said. Three of them are in critical condition and one is in serious condition, she said.

Highway patrol officers block off Herndon Avenue at Highway 99 after a natural gas explosion in Fresno, Calif., on Friday. Smoke from the fire can be seen in the background. The Associated Press

Four other patients were taken to St. Agnes Hospital and three more to Madera Community Hospital, Fresno County Medical Services director Dan Lynch said.

Traffic heading north and south on Highway 99 in Fresno was halted by the explosion about 2:30 p.m. as flames towered over the roadway, the California Highway Patrol reported. The highway’s southbound lanes were opened an hour and a half later, California Highway Patrol Officer Sean Duncan said.

The explosion happened at the Fresno County Sheriff’s gun range, where a work crew, including county jail inmates, using heavy equipment apparently hit a pipe carrying natural gas, said Tony Botti, a spokesman for the Fresno County Sheriff’s Office. One of the workers appeared to be in critical condition, he said.

No law enforcement officers were injured, Botti said. A nearby rail line was also halted out of concern that a passing train could spark leaking gas.

Witnesses reported seeing a large fireball, said Pete Martinez of the Fresno Fire Department. The flames prompted a two-alarm call of firefighters.

The 12-inch diameter pipeline involved in the fire belongs to the Pacific Gas & Electric Co., Martinez said. A front loader was in the area, but it’s unclear if the vehicle was digging at the time of the explosion. The operator was flown by helicopter to a hospital in serious condition, Martinez said.

“It was a large explosion that shook the surrounding area,” he said.

The operator of the front loader worked for the construction company, not PG&E, Martinez said.

PG&E spokesman Keith Stephens referred questions to local authorities. Stephens said he could not comment on whether the pipeline involved was PG&E’s or whether anyone with PG&E had been at the scene.

“Our thoughts and prayers are with those who are reported to be injured. We do not have definitive information” on the situation, he said.

PG&E’s natural-gas operations have been under intensifying scrutiny in the wake of a fiery 2010 PG&E pipeline blast that killed eight people in the San Francisco suburb of San Bruno.

National Transportation Safety Board investigators blamed faulty safety practices by PG&E, and lax oversight by state regulators, for the 2010 blast.

Earlier this month, state regulators leveled the state’s biggest-ever penalty against a utility – $1.6 billion – against PG&E for the San Bruno blast. California Public Utilities Commission President Michael Picker said at the time that continuing safety citations against the utility made him doubt that the utility had embraced a culture of safety, and he raised the possibility of breaking apart the utility’s gas and electric operations.

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