U.S. antitrust enforcers are investigating whether Google Inc. illegally increased advertising rates 50-fold for rival Microsoft Corp., Bloomberg reported.

The Federal Trade Commission (FTC) is probing the alleged increase, along with other allegations against Google related to advertising, as a result of complaints from Microsoft, according to a person familiar with the matter. The complaints are being examined as part of a larger antitrust probe into Google that began earlier this year, the person said.

If true, the Microsoft allegations could be used to help the FTC build a case showing that Google abused its power as the owner of the world’s most popular search engine, violating the Sherman Act and other antitrust laws, said Andre Barlow, an antitrust lawyer at Doyle, Barlow & Mazard PLLC, in an interview with Bloomberg.

“A lot of this conduct, when put together with a firm with market power, could be viewed as a violation,” he said.

The FTC will consider the motives for the accusations by Microsoft, the world’s largest software maker and one of Google’s biggest competitors, Barlow said.

A spokesman for Google, Adam Kovacevich, said that although company officials didn’t know the details of Microsoft’s allegations about ads, rates are usually determined in part by how closely related an ad is to a user’s search.

Jack Evans, a spokesman for Microsoft, confirmed that the company had made advertising complaints against Google.