The U.S. Department of Justice should take "vigorous action to stop this abusive practice," wrote Mark Cooper, director of research for the Consumer Federation of America, in a letter to the U.S. Senate Judiciary Committee's antitrust subcommittee.

Apple did not immediately respond to a request for a comment on Cooper's letter.

In December, a U.S. Department of Justice official said the . A similar investigation by the European Union has targeted Apple and five e-book publishers.

The price-setting practices by Apple and the five publishers "constitute anticompetitive, anti-consumer collusive" behavior, Cooper wrote in his seven-page letter to the subcommittee. "Collusion between firms to set minimum prices is 'slam dunk' illegal, especially when one of the first effects of the price fixing, after increasing consumer cost, has been to raise publisher profits."

A in U.S. District Court for the Northern District of California, accuses Apple, HarperCollins Publishers, Hachette Book Group, Macmillan Publishers, Penguin Group and Simon & Schuster of working together to raise e-book prices in response to a discounted pricing strategy from Amazon.com.