European Union Competition Commissioner Margrethe Vestager said Google faces a “very large investigation” into its advertising business, adding a new front to a decade-long antitrust battle.

Technology “is really a high priority for us because what has happened over these last 12 months has changed a lot of habits,” Vestager told an online event Friday.

Vestager’s investigation into the “Google ads ecosystem” is one of series of tech probes she cited, including investigations into Apple Inc.’s app store and payment system, Facebook Inc.’s marketplace and data. Her most advanced investigation into Inc. focuses on how its control of seller data may be “dramatically reducing the risks of Amazon retail compared to the risk” run by traders on the platform

Regulators gathered information in January on Google’s practices in the “advertising technology value chain” according to questionnaires sent to publishers and ad firms. The EU has been examining Google’s data practices since 2019 and has widened the scope to look at Google’s plans to phase out third-party cookies, which the U.K.’s antitrust authority is investigating amid complaints from publishers.

“We will use every tool that we have to the fullest” with antitrust action necessary “to make sure that the marketplace is fair,” Vestager said on the event organized by the European retailers’ association, EuroCommerce.

Google didn’t immediately respond to a request for comment on Vestager’s remarks.

Vestager previously said she’s taking a keen interest in Google’s advertising business in order to “fully understand” it and respond to people who “are uncomfortable as to how it works.” She said in January she didn’t know if regulators would open a formal probe to scrutinize the issues more deeply.

The Dane has already fined Google more than US$9 billion euros after three earlier antitrust investigations found the company breached the law over its shopping service, Android mobile phone operating system and advertising business. Regulators have also looked at the company’s travel and local search services.