The DoJ opened a formal antitrust probe on June 16, four days after Yahoo said it would run advertisements supplied by Google alongside search query results. The four-year deal is estimated to bring Yahoo up to $800m (£400m) in revenue a year. A DoJ spokesperson confirmed this week that the investigation has yet to be concluded.
Yahoo and Google have been keenly aware of how their partnership might draw regulatory attention. In April, the two companies informed the DoJ of a planned two-week test of the advertising program, which was limited to US traffic on yahoo.com and comprised no more than 3 percent of total search queries.
After reaching a larger agreement last month, the two companies said they would delay implementing the program for three-and-a-half months pending a review by the DoJ.
The investigation means the DoJ can ask questions of both companies, and request documents and other material needed for its probe. It can also include the issuance of 'civil investigative demands', a type of legal request for information.
Yahoo said the course of the investigation is proceeding as expected. "We cannot comment on the specific details of the process, but there is nothing unexpected in the review of this agreement as structured by the parties and Department of Justice officials," according to a statement issued by Yahoo.
"We are continuing to have cooperative discussions with the DoJ about this arrangement," Google said. "We are confident that the arrangement is beneficial to competition, but we are not going to discuss the details of the process."
The deal with Google, which already dominates text-based search advertising, was widely characterised as a move by Yahoo to deflect Microsoft's acquisition attempts.