Three dozen states and the District of Columbia filed an antitrust lawsuit against Alphabet Inc.’s Google on Wednesday, alleging that the company operates an illegal monopoly with its Google Play app store.

The bipartisan antitrust suit adds to the company’s mounting legal challenges. Led by the state of Utah and filed in the U.S. District Court for the Northern District of California, it alleges that the company has monopolized the distribution of apps on mobile devices that run the Google-owned Android operating system, blocking competition through contracts, technical barriers and other means.

In a blog post, Google said it provides an open operating system in which customers are free to download apps directly from developers’ websites.

“It’s strange that a group of state attorneys general chose to file a lawsuit attacking a system that provides more openness and choice than others,” wrote Wilson White, senior director of public policy.

Over the past year, Google has faced lawsuits from state and federal officials over its dominance of search advertising and its ad tech business.