A large gas pipeline in central California exploded Friday injuring at least 11 people, at least three of them critically, police said.

It was not immediately clear what caused the explosion at the Fresno County Sheriff’s gun range that brought traffic to a standstill. Authorities say it occurred while a country equipment operator was working with a jail inmate crew to expand on the range alongside Highway 99.

Witnesses told The Associated Press that flames shot up to well over 100 feet in the air.

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

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 was reopened three hours later, the CHP said.

Kevin Ling was driving by shortly after the blast, and he saw fire flying into the sky.

"As I got closer, the flames were just bigger and bigger," he told The Associated Press. "It was shooting up to 200 feet or more, and a fireball maybe 10 to 15 feet in diameter. It was like out of a movie."

"My window was up and my AC was on and it still felt like a furnace inside my car," he added.

Fresno County Sheriff Margaret Mims said crews using heavy equipment apparently hit a Pacific Gas & Electric Co. pipe carrying natural gas, triggering the explosion.

The front-loader driver was a county public works employee who had been working at the shooting range all day, along with 13 jail inmate workers. Mims said they were expanding a road on a tall berm that confines the range.

Asked whether the driver was scraping or digging the earth when the gas exploded, Mims said her office is investigating. á "Hopefully we'll be able to speak to the worker to see what action he was taking at the time," she said.

The flames prompted a two-alarm call of firefighters, said Pete Martinez of the Fresno Fire Department.

The 12-inch diameter pipeline involved in the fire belongs to PG&E, 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.

 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.

The Associated Press contributed to this report