SAN FRANCISCO (AP) — State regulators on Tuesday directed Pacific Gas and Electric to keep a Northern California natural gas pipeline shut down until they verify its safety after concerns surfaced in an email from an engineer.

The California Public Utilities Commission has opened an investigation to determine whether any immediate dangers are posed by the pipeline in the San Francisco Bay Area city of San Carlos, saying the findings would be made public, and PG&E was not authorized to re-energize the pipeline until the probe is completed.

"We are committed to ensuring that PG&E is operating pipelines safely," CPUC Mike Florio said in a statement. "We will not compromise safety."

The utility complied with a judge's order and on Monday shut down the 83-year-old pipeline, but maintains that it is safe despite concerns from the engineer, a contractor whose email questioned the welding on the pipeline known as Line 147.

The judge ordered the shutdown after San Carlos city officials discovered the email saying PG&E's records incorrectly show the line containing a newer, more reliable weld than it actually has, and said the city was in a state of emergency.

"We appreciate the CPUC's acting quickly to keep this line shut down," City Manager Jeff Maltbie said Tuesday.

PG&E said it welcomes the chance to demonstrate the safety of the line and the recent work done on it.

"Customers in these communities can be assured that Line 147 is safe and we look forward to the opportunity to document all the work that has gone into maintaining and operating this line safely," PG&E said in a statement.

PG&E said the contractor's email came from the sort of due diligence the company encourages, and that the leaking section of pipeline that it discussed has been repaired and withstood the rigorous safety tests instituted after a 2010 explosion of a pipeline in San Bruno 12 miles away.

That blast and the gas-fueled fire that followed led to eight deaths, destroyed 38 homes and laid waste to much of the surrounding neighborhood.