“We need a true cop-on-the-beat policing the vital oil market,” Cantwell said in a statement. “That’s why I plan on asking the Department of Justice to investigate on a refinery-by-refinery basis and get the answers consumers deserve.”

The McCullough Research report stated that oil production in California actually increased during the month of May, meaning prices shouldn't have increased because of a lack of supply and high demand. The report also said refinery fires were blamed for the price spikes, but "the lengthy delay between cause and effect makes these explanations suspect."

“Washingtonians were hit hard this year by gas price spikes supposedly caused by supply disruptions,” Cantwell said. “This report indicates that the gas price spike may have been caused by more than just supply and demand.”

According to the McCullough report, the October price spike added up to a 66-cent-per-gallon windfall profit for oil companies — or about $25 million a day. Cantwell said the difference between what drivers actually paid and what they should have paid exceeded $1 billion.

Earlier this year, Cantwell asked the Federal Trade Commission to look into why her home state’s gas prices spiked in May, while national average prices fell.