A plan to test Google's AdSense on a portion of Yahoo's U.S. search traffic has raised eyebrows at the Department of Justice, according to a Reuters report.

The test, which is set to end this week, was part of a brief experiment to determine if Yahoo could improve its revenue -- and possibly fend off Microsoft -- by outsourcing its search ads to rival Google. But that test could be a violation of U.S. antitrust laws, according to officials at the Justice Department, who confirmed that they have initiated an investigation.

Both Google and Yahoo said they informed officials at Justice about the test before it began. Neither company has said if it plans to continue the experiment.

Microsoft, which is no stranger to the intricacies of U.S. antitrust laws, did not comment on the news. However, Microsoft has been known to use Justice as something of a dagger. During the year that it took Google to close its DoubleClick deal, it was revealed that a lot of the regulatory pressure put on the search giant was a result of influence by Microsoft.