The Us Justice Department have initiated an investigation into Yahoo and Google’s advertising sharing trial. The two week test, which allows Google to place its adverts alongside 3% of Yahoo’s search results, has been criticized since it was announced. The Department of Justice is looking into whether it violates any antitrust laws.

Although The Justice Department has not made a formal announcement of the investigation, insider sources have said that the trial was being looked at because of a phone call between the two companies CEO’s, in which its alleged that Eric Schmidt (Google CEO) has offered help in thwarting Microsoft’s bid to buy Yahoo.

Another source later said that The DOJ was concerned about an extension to the deal between Yahoo and Google. Microsoft has declined to comment, with many analysts saying that the Yahoo/Google test was set up to stall Microsoft’s merger with Yahoo and so that Google could avoid a competitive threat posed by the merger.

Yahoo and Google seem to think that they are in the clear as they haven’t hidden their intentions from the DOJ, but could be liable for a monopolization inquiry, if it is deemed to have a significant competition avoiding affects.