California attorney general says he has been investigating Facebook's privacy practices since 2018.
He's asking a court to force Facebook to answer its subpoenas. He says Facebook has failed to respond adequately to subpoenas.
The investigation is into Facebook's practices related to privacy, disclosures and third-party access to user data.
Officials say the probe began as a response to the Cambridge Analytica scandal but has expanded.
California hadn't joined a separate probe involving attorneys general from New York and other states. The New York probe is looking into Facebook's dominance and any resulting anticompetitive conduct.