FRESNO, Calif. -- A large gas pipeline exploded into a tower of fire Friday in California, closing both directions of a major highway in the region and injuring at least 11 people, at least three of them critically, authorities said.

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 a work crew that included jail inmates was working with heavy equipment at the time.

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, and four others were treated and released at the scene, 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.

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 Pacific Gas & Electric Co. pipe carrying natural gas, said Tony Botti, a spokesman for the Fresno County Sheriff's Office.