Democratic U.S. Sen. Amy Klobuchar of Minnesota has joined with Republican colleague Sen. Charles Grassley of Iowa to push federal investigations of alleged restrictions on the sales of ethanol by the nation's major oil companies.

The senators cited a recent report by the Renewable Fuels Association, a trade group representing the ethanol industry, that claims name-brand oil companies unfairly limit sales of ethanol at service stations selling their products.

Klobuchar and Grassley have written to U.S. Atty. Gen. Eric Holder and Federal Trade Commission Chairwoman Edith Ramirez asking them to investigate a number of charges for possible legal and regulatory violations. The senators have asked for "a substantive evaluation of your conclusions regarding possible anticompetitive behavior by certain oil companies and any proposed solutions or actions the DOJ and FTC will take to resolve this issue."

Among charges leveled by the renewable fuels group at Big Oil:

Brand name service stations can only sell products provided by the oil company.

Sales quotas of branded products discourage the sale of ethanol.

Requirements to store multiple grades of branded gas eliminate the ability to store and sell ethanol.

Oil company demands that ethanol pumps be labeled with "intimidating" warnings about how the fuel can hurt engines.

Forcing dealers to isolate E85 pumps that deliver fuel that is 85 percent ethanol.