FTC Chairman Jon Leibowitz said the agency does not confirm investigations "but we do have enormous respect for Sen. Feinstein."
Feinstein has asked the FTC to determine if the price spike was caused by illegal manipulation of the market and to start monitoring the market for fraud, manipulation, or other malicious trading practices.
"Publically available data appears to confirm that market fundamentals are not to blame for rising gas prices in California," she wrote.
Despite a pipeline and refinery shut down, she said, state data shows gas production last week was "almost as high as a year ago, and stockpiles of gasoline and blending components combined were equal to this time last year," she said.
In some locations, fuming motorists paid $5 or more per gallon while station owners had to shut down pumps in others.
A station in Long Beach south of Los Angeles had California's priciest gas at $6.65 for a gallon of regular, according to GasBuddy.com. Meanwhile, customers at an outlet in San Pablo north of Oakland paid just $3.49, the lowest.
Winter-blend gas typically isn't sold until November, when cooler temperatures allow for its use while maintaining federal and state air quality standards. Few refineries outside the state are currently making summer-blend gas, putting the pressure on already-taxed California manufacturers.
Still, the air board said it believed that changing over a few weeks earlier than normal would not push California's air pollution beyond normal levels for this time of year.
David Pettit, an air quality attorney for Natural Resources Defense Council, said concerns over air pollution from the winter gas could arise if California experiences an October heat wave.
"But right now, I don't have any great concerns about it, based on average temperatures in the past in October," he said.
Jason Dearen can be reached at http://www.twitter.com/JHDearen . Associated Press writers Christopher Weber in Los Angeles and Juliet Williams in Sacramento, and Business Writer Marcy Gordon in Washington contributed to this report.