SACRAMENTO — Gov. Gavin Newsom asked the California attorney general’s office Monday to investigate whether a “mystery surcharge” on gasoline prices in the state was caused by oil companies “misleading and overcharging” their customers.

“There is no identifiable evidence to justify these premium prices,” Newsom wrote in a letter to Attorney General Xavier Becerra. “If oil companies are engaging in false advertising or price fixing, then legal action should be taken to protect the public.”

Newsom pointed to an analysis by the California Energy Commission that could not entirely explain why gas prices are higher in the state than elsewhere. Even accounting for taxes and other production factors, the commission found customers at name-brand gas retailers such as Chevron, Shell and 76 paid an average of 30 cents more per gallon in California.

The governor called it a “mystery surcharge.”

The commission’s report said its investigators “concluded that the primary cause of the residual price increase is simply that California’s retail gasoline outlets are charging higher prices than those in other states.”

Newsom requested the analysis in April, as gas prices soared above $4 per gallon in California.

Much of the difference in California’s higher prices can be attributed to extra costs from environmental regulations, such as a low-carbon requirement designed to cut greenhouse gas emissions and a cap-and-trade program for polluters. State taxes, including new fees to fund road repairs that took effect in 2017, are also a factor.

But the Energy Commission, which released its analysis Monday, found that the margin between retail gasoline prices and wholesale costs rose at a faster rate in California in recent years than it did in the rest of the country.

That spike began with a major refinery outage in Torrance (Los Angeles County) in 2015, the commission said, and has continued ever since, even as production levels returned to normal. The commission estimated that the unexplained price surge has cost California consumers an additional $11.6 billion.

The commission noted that prices were significantly higher at name-brand gas stations such as Chevron, Shell and 76 that purport to sell a higher-quality gasoline, though investigators could find no evidence of a difference in quality from other retailers. All gasoline in California must meet specific requirements for added ingredients set by the California Air Resources Board.

“The name-brand stations, therefore, are charging higher prices for what appears to be the same product,” the commission concluded.

However, the commission did not determine that the companies were falsely advertising a superior quality of their gasoline or that the industry was arranging to collectively fix prices. The analysis found that expensive brands had maintained their share of the gasoline market in California even as the companies raised their prices more than other retailers.

“Consumers may be purchasing higher-priced gasoline brands for convenience, credit card acceptance, or other reasons,” the commission wrote, and referred the matter to the state Department of Justice for further investigation.

The attorney general’s office confirmed that it had received Newsom’s request and would “handle accordingly.”

A spokesman for Shell said the company had no comment on the report. Phillips 66, a Texas energy company that owns 76, did not respond to a request for comment. Chevron directed inquiries to the Western States Petroleum Association, a trade group for oil companies in California and four other states.

“It’s important to note that (the Energy Commission’s) own numbers show our state’s regulatory environment plays a big role in the ever-increasing affordability challenges Californians face,” Catherine Reheis-Boyd, president of the association, said in a statement. “In fact, the first $1.07 per gallon at the pump is a result of taxes and California’s regulatory programs, such as the low carbon fuel standard.”

California gas prices recently climbed again to an average of more than $4, sitting at $4.26 per gallon in San Francisco on Monday, according to AAA. That compares with a national average of $2.65.

Analysts have blamed refinery maintenance in the state, low national production and global economic uncertainty for the latest price increases.

Alexei Koseff is a San Francisco Chronicle staff writer. Email: alexei.koseff@sfchronicle.com Twitter: @akoseff