(AP) -- The Senate Commerce Committee is investigating several little-known companies for charging consumers mystery fees through their phone bills in a practice known as cramming.

Senate Commerce Committee Chairman John D. Rockefeller, D-W. Va., sent letters to three companies Friday asking them to explain unauthorized charges that they or affiliates have placed on landline phone bills. Those charges, Rockefeller said, were often for services that are available for free online, such as e-mail, or are already bundled with regular phone service, such as voice mail.

The letters went to daData Inc. of Palm Harbor, Fla.; My Service and Support of New Hope, Minn.; and More International of Carson City, Nev. None of the companies could be reached for comment.

The Senate inquiry was made public on Friday. In June, the committee sent letters to AT&T Inc., Verizon Communications Inc. and Qwest Communications International seeking information about the scope of the cramming problem, their efforts to address it and a list of companies that place third-party charges on their customers' phone bills.

Although the inquiry in still ongoing, the Commerce Committee has already found that many of the companies responsible for these third-party charges face cramming complaints filed by consumers with the Better Business Bureau, the Federal Trade Commission and the Federal Communications Commission. The probe has also found that many of these companies appear to be controlled by just a few entities.

Senate investigators are still looking into how these companies obtain consumers' phone numbers and how they get people to consent to purchase the services in question. The committee is also looking into the relationships among different companies engaged in cramming.

For its part, AT&T said it is working with Rockefeller to address cramming. Qwest said it has measures in place to combat the problem, including a dispute resolution procedure to issue adjustments if customers dispute third-party charges on their bills. Verizon did not immediately return a message seeking comment.

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