U.S. District Judge Leonie Brinkema in Alexandria, Virginia, ruled that Google unlawfully monopolized markets for publisher ad servers and the market for ad exchanges which sit between buyers and sellers. Antitrust enforcers failed to show the company had a monopoly in advertiser ad networks, she wrote.
The ruling could allow prosecutors to argue for a breakup of Google's advertising products. The U.S. Department of Justice has said that Google should have to sell off at least its Google Ad Manager, which includes the company's publisher ad server and its ad exchange.
Google will now face the possibility of two different U.S. courts ordering it to sell assets or change its business practices. A judge in Washington will hold a trial next week on the DOJ's request to make Google sell its Chrome browser and take other measures to end its dominance in online search.
Google has previously explored selling off its ad exchange to appease European antitrust regulators, Reuters reported in September.
Brinkema oversaw a three-week trial last year on claims brought by the DOJ and a coalition of states.
Google used classic monopoly-building tactics of eliminating competitors through acquisitions, locking customers in to using its products, and controlling how transactions occurred in the online ad market, prosecutors said at trial.
Google argued the case focused on the past, when the company was still working on making its tools able to connect to competitors' products. Prosecutors also ignored competition from technology companies including Amazon.com (AMZN.O) and Comcast (CMCSA.O) as digital ad spending shifted to apps and streaming video, Google's lawyer said.
