It was not clear what caused the explosion at the Fresno County Sheriff's gun range that brought traffic in the area to a halt. But authorities say it occurred while a county equipment operator was building up a dirt berm on the range alongside heavily travelled Highway 99.

The flames shot well over 100 feet into the air, witnesses 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.

The explosion on a Pacific Gas & Electric Co. pipe carrying natural gas happened at the gun range while an equipment operator built a dirt berm to confine gunfire to the range, the Fresno Sheriff's Department said. A group of county jail inmates were doing cleanup work nearby.

The driver of the front-loader was a county public works employee who had been working at the shooting range all day, according to Fresno County Sheriff Margaret Mims.

Ten inmates and the operator were hospitalized, she said. Three inmates were e valuated and sent back to jail, and two deputies were being evaluated for ringing ears and exposure to the hot blast.

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. Four other patients were taken to St. Agnes Hospital and three more to Madera Community Hospital.

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

Mims noted PG&E had, some time ago, marked the location of gas lines in the construction zones.

The flames prompted a two-alarm call of firefighters, said Pete Martinez of the Fresno Fire Department. A nearby rail line was also halted out of concern that a passing train could spark leaking gas.

Freelance photographer Kevin Ling, 42, was driving by shortly after the blast when he saw fire flying into the sky.

"As I got closer, the flames were just bigger and bigger," he said. "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.

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

PG&E spokesman Denny Boyles said the pipeline was damaged by a vehicle and that the line was buried but he didn't know how deeply. The flame from the pipe was extinguished at about an hour and a half after the blast, he said.

Another PG&E spokesman, Keith Stephens, referred questions to local authorities.

"Our thoughts and prayers are with those who are reported to be injured," he said.

The California Public Utilities Commission said in a statement that it is investigating the explosion in cooperation with the Federal Pipeline and Hazardous Materials Safety Administration.

PG&E's natural-gas operations have been under scrutiny following 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.