WASHINGTON >> Google acted illegally to maintain a monopoly in some online advertising technology, a federal judge ruled Thursday, adding to legal troubles that could reshape the $1.88 trillion company and alter its power over the internet.

Judge Leonie Brinkema of U.S. District Court for the Eastern District of Virginia said in a 115-page ruling that Google had broken the law to build its dominance over the largely invisible system of technology that places advertisements on pages across the web. The Justice Department and a group of states had sued Google, arguing that its monopoly in ad technology allowed the company to charge higher prices and take a bigger portion of each sale.“In addition to depriving rivals of the ability to compete, this exclusionary conduct substantially harmed Google’s publisher customers, the competitive process, and, ultimately, consumers of information on the open web,” said Brinkema.

The government argued in its case that Google had a monopoly over three parts of the online advertising market: the tools used by online publishers, like news sites, to host open ad space; the tools advertisers use to buy that ad space; and the software that facilitates those transactions.

Brinkema ruled in the government’s favor in two of those, finding that Google illegally built a monopoly over the publisher tools and the software system. She dismissed the third, the tools used by advertisers, saying the government had failed to prove that it constituted a real and defined market.

Google has increasingly faced a reckoning over the dominant role its products play in how people get information and conduct business online. Another federal judge ruled in August that the company had a monopoly in online search. He is now considering a request by the Justice Department to break the company up, with a three-week hearing on the matter scheduled to begin Monday.

Brinkema, too, will have an opportunity to force changes to Google’s business. In its lawsuit, the Justice Department preemptively asked the court to force Google to sell some pieces of the ad technology business it had acquired over the years. The government will now assess the ruling to determine what to ask the court to do to remedy the monopoly.

Together, the two rulings and their remedies could check Google’s influence and result in a major restructuring of the company.

“We won half of this case, and we will appeal the other half,” said Lee-Anne Mulholland, Google’s vice president of regulatory affairs. “Publishers have many options, and they choose Google because our ad tech tools are simple, affordable and effective.”

The Justice Department did not comment.

“This is a landmark victory in the ongoing fight to stop Google from monopolizing the digital public square,” said U.S. Attorney General Pamela Bondi.

The cases against Google are part of a push by regulators to rein in the power of big tech companies, which shape commerce, information and communication online. The Justice Department has sued Apple, arguing that the company made it difficult for consumers to leave its universe of devices and software. The Federal Trade Commission has sued Amazon, accusing it of squeezing small businesses, and Meta, for killing rivals when it bought Instagram and WhatsApp. The trial against Meta started this week.

President Donald Trump has signaled that his administration will continue taking a tough stance on antitrust for the tech industry, despite efforts by tech executives to court his favor. His choices for FTC chair and the Justice Department’s top antitrust role have said they intend to look closely at the power that tech companies have over online discourse. The Google search case was brought under his first administration.

“Many of these cases germinated — and one of them was filed — during the first Trump Administration, and they were aggressively pursued throughout the Biden Administration,” said UC Berkeley law school adjunct professor and professional mediator Christopher Hockett, an antitrust expert who has closely followed the anti-monopoly cases against Google. “There are no obvious signals that the enforcers are going to back off now.”

The scrutiny over Google and other dominant tech companies isn’t limited to the current Republican administration. Democratic attorneys general Rob Bonta of California and Leticia James of New York were among 17 to join the federal case.

“Advertising is key to a business’s success, and Google has been playing unfairly in the advertising space,” Bonta said in a statement Thursday. “As the fifth largest economy in the world, California has an outsized role in protecting competition and a vibrant economy where business can thrive on merits, not on illegal business practices -- today, we’ve done just that.”

The ad tech case — U.S. et al. v. Google — was filed in 2023 and concerns an intricate web of programs that sell ad space around the web, like on a news site or a recipes page. The suite of software, which includes Google Ad Manager, conducts split-second auctions to place ads each time a user loads a page. That business generated $31 billion in 2023, or about 10% of the overall revenue for Google’s parent company, Alphabet.

Part of that business stems from the acquisition of DoubleClick, an advertising software company, for $3.1 billion in 2008. Google now has an 87% market share in ad-selling technology, according to reports.

The government argued during a trial in September that Google had a monopoly over multiple pieces of technology that are used to conduct these transactions. The company locked publishers into using its software and was able to take more money off the top of each transaction, the government said.

That hurt websites that produce content and make it available online for no charge, the government said.

Hockett said the Justice Department will almost certainly urge the court to order Google to divest itself of its DoubleClick for Publishers and its Ad Exchange businesses. The department may also seek a court order forcing Google to provide divested businesses with resources so they can compete independently, and an order that Google make up for harm its monopoly has caused, Hockett said.

“Depending on the remedy that the court orders, it could be very disruptive,” Hockett said. Dismantling Google’s integrated advertising technology services “would likely impose significant costs and complications,” he added.

Staff Writer Ethan Baron contributed to this report.