California is investigating Facebook's privacy practices and has accused the social media giant of stonewalling investigators.
The 18-month-old probe came to light Wednesday in a court filing demanding Facebook respond to a subpoena sent in June asking for internal documents.
Facebook had no immediate comment.
The investigation by state Attorney General Xavier Becerra is the latest bad news for the Menlo Park tech firm, which is already under investigation by 47 U.S. states.
Becerra's investigators asked the court to require Facebook to turn over information about developers and other third parties' access to data.
They are also seeking records of communications between founder and CEO Mark Zuckerberg and his chief operating officers.
The filing states the investigation grew out of a probe into the revelation that in 2013 Facebook exposed the data of tens of millions of users.
That data that was scraped up by consulting firm Cambridge Analytica, which put it to work for Donald Trump's 2016 presidential campaign.
California's court filing says that an initial probe into the Cambridge Analytica scandal "expanded over time to an investigation into whether Facebook has violated California law, by among other things, deceiving users and ignoring its own policies in allowing third parties broad access to user data."
The filing paints Facebook as a company that allowed app developers to access non-public information about its users.
Those developers were able to "build profiles on users, and sell those to third parties."
Facebook is also under investigation by the U.S. Department of Justice, the Federal Trade Commission, and the U.S. Congress.