Sens. Claire McCaskillClaire Conner McCaskillKoch-backed group targets red-state Dems on tax reform Overnight Cybersecurity: Equifax security employee left after breach | Lawmakers float bill to reform warrantless surveillance | Intel leaders keeping collusion probe open Las Vegas highlights Islamist terrorism is not America's greatest domestic threat MORE (D-Mo.), Charles SchumerCharles (Chuck) Ellis SchumerOvernight Health Care: Schumer calls for tying ObamaCare fix to children's health insurance | Puerto Rico's water woes worsen | Dems plead for nursing home residents' right to sue Crying on TV doesn't qualify Kimmel to set nation's gun agenda Trump knocks ‘fake’ news coverage of his trip to Puerto Rico MORE (D-N.Y.), Dick DurbinRichard (Dick) Joseph DurbinGun proposal picks up GOP support Durbin: I had 'nothing to do' with Curbelo snub Republicans jockey for position on immigration MORE (D-Ill.) and Patty MurrayPatricia (Patty) Lynn MurrayChildren’s health-care bill faces new obstacles Overnight Health Care: Schumer calls for tying ObamaCare fix to children's health insurance | Puerto Rico's water woes worsen | Dems plead for nursing home residents' right to sue Schumer calls for attaching ObamaCare fix to children's health insurance MORE (D-Wash.) cited press reports that this may be happening and told FTC Chairman Jon Leibowitz that this would be a "direct affront" to American consumers.

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"At a time when major refiners and oil companies are making record profits and American families continue to struggle with gasoline at record prices, the idea that refiners may be manipulating the market to keep prices artificially high is offensive," they wrote.

"It is incumbent upon the Commission to ensure that the American people are protected from this type of manipulation," the letter continued. "Accordingly, we request that the Commission open a full investigation into these allegations of wrongdoing and to determine the impact this behavior, if confirmed, has on regional and national gasoline prices."

The letter acknowledged that rising oil prices are "certainly a driving factor" behind gas price increases. However, it added that U.S. gasoline use is down, but gasoline inventories are also low, and refineries are using only 81.7 percent of their capacity, down 7 percent from last year.

They also said refiners have seen a 90 percent increase in their margins. "While some have argued that this increase is due to potential impacts from recent flooding along the Mississippi River, this cannot justify the steady increases in their margins since January of this year," they wrote.

McCaskill noted the letter on the Senate floor Tuesday, just before a planned evening vote to end debate on a bill that would raise taxes on oil companies.