FRESNO, Calif. » Pacific Gas & Electric Co. investigators were at the scene Saturday of a natural gas pipeline blast at a sheriff’s gun range, as four of the 11 people injured in the explosion were released from the hospital.

The victims included two Fresno County sheriff’s deputies, a public works employee and eight jail inmates, according to sheriff’s spokesman Tony Botti.

The deputies and two of the inmates were released. The public works employee and another inmate were in critical condition, with the inmate’s condition considered life-threatening, Botti said.

Friday’s explosion on a PG&E pipe occurred at the Fresno County sheriff’s gun range while the county employee was using a front-loader to build a dirt berm to confine gunfire to the range, sheriff’s officials said.

A group of county jail inmates was doing cleanup work about 50 feet away, Botti said.

The explosion sent flames towering over Highway 99 and warped 400 feet of nearby railway line, shutting both down.

It was not clear whether the front-loader was being used to dig at the time of the explosion, fire and sheriff’s officials said.

PG&E’s natural-gas operations have been under scrutiny since a fiery 2010 PG&E pipeline blast 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.

PG&E said the pipeline in Friday’s incident was buried and was damaged by a vehicle. It had been surveyed in 2013, and there was about 40 inches of cover between the pipe and the surface in accordance with PG&E standards and federal code, the company said.

It was not notified in advance of any work in the area, spokesman Nick Stimmel said. Such advance notice allows PG&E to mark any gas lines.

Sheriff Margaret Mims said PG&E had previously marked the location of gas lines in the construction zones.

PG&E crews planned to cap the line on Saturday so they could remove the damaged section, Stimmel said.

Crews were also expected on Saturday to complete repairs and reopen the freight rail line that was damaged, Union Pacific spokesman Francisco Castillo said.

The California Public Utilities Commission said it was investigating the explosion in cooperation with the federal Pipeline and Hazardous Materials Safety Administration.

Rodriguez reported from San Francisco. Associated Press writers Paul Elias, Ellen Knickmeyer and Sudhin Thanawala in San Francisco and Andrew Dalton in Los Angeles contributed to this story.