The two sides made their final cases to a federal judge Monday in a trial over the tech giant’s dominance in technology that sells ads online.

Lawyers for the United States on Monday said that Google had created a monopoly with its services to place ads online, closing out an antitrust trial over the company’s dominance in advertising technology that could add to the Silicon Valley giant’s mounting woes.

The legal case concerns a system of software that is used by advertisers to place ads on websites around the internet. Aaron Teitelbaum, a lawyer for the Justice Department, told Judge Leonie M. Brinkema of the U.S. District Court for the Eastern District of Virginia that the company had linked its products together in a way that made it hard for publishers and advertisers to use alternatives.

“Google is once, twice, three times a monopolist,” he said. “These are the markets that make the free and open internet possible.”

Google’s lead lawyer, Karen Dunn, countered that the government had failed to offer the evidence to prove its case and was on shaky legal ground.

“Google’s conduct is a story of innovation in response to competition,” she said.

The arguments conclude U.S. et al. v. Google, an antitrust suit that the Justice Department and eight states filed against Google last year. (More states have joined the suit since then.) The agency and states accused the internet giant of abusing control of its ad technology and violating antitrust law, in part through the acquisition of the advertising software company Doubleclick in 2008. Next, Judge Brinkema will decide the merits of the case in the coming months.

Google has been under pressure on multiple fronts for its towering influence across technology markets and whether it has illegally wielded its power to crush competition. In recent years, the Justice Department has brought multiple antitrust cases against the company. In August, a federal judge issued a landmark ruling in one case involving online search, finding that Google had broken antitrust laws to maintain its dominance in an arena where it is so ubiquitous that it is also a verb.

The penalties could be severe. Last week, the Justice Department asked the judge in that case to break Google up by forcing it to sell Chrome, the most popular web browser in the world. Google has until Dec. 20 to respond with its own suggestions on how to fix its search monopoly.

The ad-technology case could deliver a new blow to Google. The government has pre-emptively called for a breakup of the company’s ad-tech business, which generated $31 billion in revenue last year, or about a tenth of overall revenue. If Judge Brinkema decides the company has illegally subverted competition in the ad-technology market, she could force Google to spin off the technology and place other restrictions on its business.

Still, the ad-technology unit is only a portion of Google’s advertising arsenal, which also includes search ads, YouTube video ads and paid promotions on Google Maps.

Judge Brinkema’s ruling could influence a raft of other lawsuits filed against the biggest tech companies. The Justice Department has also sued Apple over competition issues, while the Federal Trade Commission has sued Amazon and Meta for anticompetitive behavior and stifling rivals.

In the Google ad-technology case, the government argued during a three-week trial in September that the company had an 87 percent market share in ad-selling technology, allowing it to take a bigger portion of each sale, which harmed news publishers and other website owners.

Judge Brinkema also heard testimony from witnesses including Google executives like the YouTube chief Neal Mohan and veterans of publishers like News Corp.

Mr. Teitelbaum, the lawyer for the Justice Department, said on Monday that Google had acted as a predator, tying together its different technologies to enrich itself at the expense of the businesses that used it.

Judge Brinkema asked several questions during Monday’s closing arguments and raised concerns about policies at Google that resulted in the deletion of some internal communications. But she did not offer clear signals about how she might ultimately rule.

In one hypothetical, she asked Mr. Teitelbaum what would happen if a company had produced the best product, and so every customer decided to use it instead of its rival’s products — which she said could be, in a sense, anticompetitive.

Mr. Teitelbaum responded that she was describing competition on the merits.

“The problem is Google hasn’t done that,” he said, adding that products that could have ultimately challenged Google’s dominance had sometimes “died out” because of the search giant’s behavior.

Google has said it faces plenty of competition in the advertising-technology market, including from new players like social media platforms.

Ms. Dunn, Google’s lawyer, said at the beginning of her closing argument that “the law simply does not permit, and the evidence does not support,” how the government is asking the court to rule in the case.

She said that Google had come up with innovative products. Over the period of time at issue in the case, prices had come down, the amount of money being spent on ads had gone up and quality had improved.

Ms. Dunn called it “the exact opposite” of the proof needed to make a case against a monopoly.

Julia Tarver Wood, the government’s lead lawyer, handled the Justice Department’s rebuttal to Ms. Dunn’s arguments. She said one of Google’s witnesses had suggested that publishers could build their own competitors to Google’s products, but she countered that it would be extremely expensive for them to do so.

The Justice Department had already proved it could be successful in its efforts to challenge the biggest players in corporate America, said Bill Kovacic, a former chairman of the Federal Trade Commission. It would build on that reputation if it won against Google in both search and ad-tech challenges.

“This would be still more impressive for them to get both,” he said. “But they got one, and one was essential for this program to be credible.”