Alexandria. Google, already facing the possibility of a breakup over its dominant search engine, is now defending itself against a new antitrust challenge from the US Department of Justice. This time, the focus is on Google's advertising technology, which places ads in front of online consumers.

The Justice Department and Google are set to make closing arguments Monday (Tuesday morning in Indonesia) in a trial that alleges Google's advertising technology amounts to an illegal monopoly. US District Judge Leonie Brinkema, in Alexandria, Virginia, will ultimately decide the case, with a ruling expected by the end of the year. If the judge finds that Google has engaged in monopolistic practices, further hearings will be held to determine appropriate remedies.

The Justice Department, along with a coalition of states, has already suggested that Google should be forced to sell its ad tech business, which generates billions of dollars annually for the company.

After a month of trial testimony earlier this year, the arguments in the case remain unchanged. The Justice Department asserts that Google has built and maintained a monopoly in "open-web display advertising"—the rectangular ads that appear on the top and sides of web pages.

Google dominates all aspects of the market: its "DoubleClick" technology is used by news sites and other online publishers, while "Google Ads" connects advertisers to consumers via targeted placements. Another Google product, AdExchange, conducts near-instantaneous auctions matching advertisers to publishers.

In court documents, Justice Department lawyers argue that Google is more concerned with preserving its monopoly on advertising technology than serving its customers—both publishers and advertisers. As a result, content providers and news organizations have not been able to generate the online revenue they deserve due to Google's excessive fees for brokering transactions between advertisers and publishers, the government says.

Google counters that the government's case focuses too narrowly on online display ads. The company argues that if the broader online advertising landscape—including social media, streaming TV, and app-based advertising—is considered, Google controls only about 25 percent of the market, a share that is declining as competition intensifies.

In court filings, Google claims that the lawsuit is driven by complaints from a few rivals and large publishers. The company also defends its billions of dollars in investments to develop technology that efficiently matches advertisers with consumers, asserting that it should not be forced to share its technology with competitors.

"Requiring a company to re-engineer its technology to accommodate competitors on their preferred terms has never been mandated by U.S. antitrust law," Google wrote in its court filings.

This case is separate from a lawsuit against Google in Washington, DC, over its search engine, where a judge has ruled that the search engine constitutes an illegal monopoly but has not yet determined the remedy. The Justice Department recently announced it would seek to force Google to sell its Chrome web browser, along with other penalties. Google has criticized this request as excessive and disconnected from legitimate antitrust concerns.