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 a monopoly over multiple parts of that system of technology.

“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.

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.