Alphabet-owned Google is under investigation for alleged antitrust law violations in regard to its search practices on mobile platforms, ramping up regulatory pressure on the U.S. technology giant.

According to the Japan Fair Trade Commission, Google made agreements with Android smartphone makers to share search-related revenue if the manufacturer didn’t install a rival search engine.

Also, the regulator is investigating whether Android phone users receive priority access to Google services.

Third-party opinions are being requested by the Japan FTC as part of the investigation.

A Google spokesperson responded that Android is an “open-source platform that has enabled a diversity of partners and device manufacturers.”

The company’s openness and flexibility ensure that users can customize their devices to suit their needs, including how they browse the web, search for apps, or download them, a spokesperson told by email on Monday.

Approximately 80% of smartphones are powered by Google’s Android mobile operating system, making it the world’s most popular mobile operating system.

Google’s Android business practices have been scrutinized by regulators around the world in recent years. A record fine of 4.34 billion euros ($4.6 billion) was imposed on Google by the European Union in 2018 for abusing the dominance of the Android operating system. By requiring smartphone makers to pre-install Google apps Chrome and Search in a bundle with its app store, Google Play, the EU said Google favored its own services unfairly.

Following an appeal by Google last year, a European Union court reduced the fine somewhat, but generally sided with regulators’ findings.