Elon Musk’s top outside attorney sent Microsoft Corp. Chief Executive Officer Satya Nadella a letter Thursday, saying Microsoft had accessed Twitter’s data for unauthorized purposes.

Microsoft may have been in violation of multiple provisions of its data agreement with Twitter “for an extended period of time,” Quinn Emanuel attorney Alex Spiro wrote in a letter seen by Bloomberg News.

Microsoft’s agreement with Twitter includes the obligation to notify Twitter of any modifications to the company’s API, or application programming interface, according to Spiro’s letter. Microsoft uses Twitter’s API in several products, including Xbox One, Bing Pages, Azure and Power Platform.

But last month, the Redmond, Washington-based company said its social media planning and scheduling tools for advertisers would no longer support Twitter. Musk at the time responded by accusing Microsoft of “illegally using Twitter data” and threatening a lawsuit.

Microsoft said in an emailed statement it had received Twitter’s letter and will review the questions. “We look forward to continuing our long term partnership with the company,” it said.

Musk, who is also CEO of Tesla Inc. and SpaceX, bought Twitter for $44 billion in October. Since then, he has slashed staff and stiffed vendors amid declining advertising revenue.

The letter, reported earlier by the New York Times, comes as Twitter changes the company’s policy to charge businesses for access to its stream of data.

Musk has also publicly criticized Microsoft’s investment in ChatGPT maker OpenAI, which the billionaire co-founded before leaving in 2018.