FRESNO, Calif. (AP) -- A natural gas pipeline explosion at a California sheriff's gun range shot flames well over 100 feet into the air, left 11 people injured and brought traffic on a busy highway to a halt, authorities and witnesses said. The explosion on a Pacific Gas & Electric Co. pipe happened at the Fresno County Sheriff's gun range while an equipment operator was using a front-loader to build a dirt berm to confine gunfire to the range, the sheriff's department said. A group of county jail inmates were doing cleanup work nearby and most were injured in the blast. Traffic heading north and south on Highway 99 in Fresno was halted by the explosion at about 2:30 p.m. as flames towered over the roadway, the California Highway Patrol reported. 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, said Pete Martinez of the Fresno Fire Department. It's unclear if the front-loader was being used to dig at the time of the explosion, he said. The driver of the front-loader was a county public works employee who had been working at the shooting range all day, Fresno County Sheriff Margaret Mims said. Ten inmates and the operator were hospitalized, she said. Four patients were being treated at Community Regional Medical Center's burn and trauma unit, spokeswoman Mary Lisa Russell said. Three of them were in critical condition and one was in serious condition, she said. The other seven injured were taken to two other hospitals. Mims noted PG&E had, some time ago, marked the location of gas lines in the construction zones. Asked whether the driver was scraping or digging the earth when the gas exploded, the sheriff 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. 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 about an hour and a half after the blast, he said. Highway 99 was reopened about three hours after the blast, the California Highway Patrol said. A nearby rail line was also halted out of concern that a passing train could spark leaking gas, Martinez 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. ___ Rodriguez reported from San Francisco. Associated Press writers Paul Elias and Ellen Knickmeyer in San Francisco and Andrew Dalton in Los Angeles contributed to this story.