SAN FRANCISCO — State regulators slapped PG&E with a $5.45 million fine Friday for using unqualified contractors to inspect its natural gas system, which raised fresh worries about the gas network’s safety in the wake of a fatal explosion in San Bruno.

An estimated 101 unqualified contractors conducted roughly half a million corrosion inspections throughout PG&E’s service territory in northern and central California, according to the state Public Utilities Commission, which fined PG&E.

The unqualified contractors’ inspections occurred between February and May, 2014, and in November, 2014 — years after a gas pipeline exploded in September, 2010, killing eight people and leveling a neighborhood in San Bruno.

“PG&E violated federal and state law by using contractors who were not properly trained or qualified, failing to check qualifications of contractors, and not having quality control and quality-assurance controls in place to ensure that its contractors were qualified,” the PUC said Friday.

Federal officials had blamed the lethal blast in San Bruno on a combination of PG&E’s shoddy maintenance and flawed record keeping, along with lazy oversight by the state PUC.

“Even though PG&E discovered the system-wide operator qualification deficiency of its contractor inspectors in 2015 and early 2016, it did not report it to the (state PUC) Safety and Enforcement Division until Sept. 14, 2016,” according to the official citation, filed Friday with the commission.

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The unqualified inspectors were checking the system for atmospheric corrosion, or corrosion on the outside of the pipes.

PG&E has begun re-inspecting the pipelines that the unqualified inspectors had checked.

“More than 18,000 severe corrosion problems” were discovered as a result of the re-inspections, the PUC’s safety enforcement division stated in a report.

The initial flawed inspections occurred primarily in Santa Clara County, Contra Costa County, San Mateo County, San Francisco, Solano County, Sacramento County, El Dorado County, Glenn County, Nevada County, Placer County, Sutter County, Yolo County and Yuba County, according to the PUC.

Most of the re-inspections have been completed, the PUC said. The rest should be finished by mid-2017.

“The PUC’s safety and enforcement division will continue to monitor these corrective actions to ensure compliance,” the state agency said.

In August, a federal jury convicted PG&E of six criminal charges for illegal actions before and after the San Bruno explosion. The jury’s guilty verdicts included one charge that the utility had deliberately obstructed a probe into the blast by the National Transportation Safety Board.

A federal judge is scheduled to sentence PG&E in January on the six convictions.

“It is sad, frustrating and dangerous that PG&E hired unqualified inspectors and then kept it from the PUC,” said state Sen. Jerry Hill, whose San Mateo County district includes San Bruno.

PG&E said it places the safety of its customers above all other considerations.

“Before this issue was self-reported to the PUC, PG&E had changed its operator-qualifications process as it pertains to the atmospheric corrosion inspections work to ensure this does not happen again,” PG&E spokesman Donald Cutler said.

The utility’s violations did not result in any injuries or property damage, the PUC safety division stated in its report about the improper inspections.

Sen. Hill said it’s particularly troubling that the utility waited a considerable time before it notified its regulator about these latest blunders linked to its aging and vast network of natural gas pipelines.

“PG&E keeps saying it has its act together, but they keep failing,” Hill said. “You really have to wonder what PG&E has learned.”