Elon Musk's lawyer sent a letter to Microsoft accusing it of misusing Twitter's data after retrieving 26 billion tweets in a year

• Elon Musk's lawyer wrote to Microsoft's CEO on Thursday, per the New York Times.
• The Verge's copy of the letter shows a dispute over Microsoft's use of Twitter's API.

Elon Musk's lawyer sent a letter to the Microsoft CEO on Thursday accusing the company of misusing Twitter's data, The New York Times first reported.

A copy of Alex Spiro's letter obtained by The Verge shows the dispute surrounds Microsoft's use of the Twitter API – a programming tool which lets external companies access the social media platform's data.

It's essential for automated Twitter accounts like the emergency alerts posted by weather services and transport authorities, but may also be used to analyze tweets.

Spiro said Microsoft operated eight apps with the API, but it "may have been in violation of multiple provisions of the agreement for an extended period of time."

While the terms of use prohibit "excessive or abusive usage," Spiro said that Microsoft's apps retrieved over 26 billion tweets just last year, per The Verge.

He wrote that Twitter therefore requests an audit of Microsoft's API use, describing all the Twitter content that was retained and how it was used.

Twitter caused a storm in March when it rolled out payment tiers for its API, which was previously free to all users. While smaller limits are still free, New York's Metropolitan Transport Authority told Bloomberg it would've had to pay $50,000 a month for access. It returned to the platform earlier this month after Twitter reversed course, per Fortune.

A Microsoft spokesperson confirmed to Insider that it "heard from a law firm representing Twitter with some questions about our previous use of the free Twitter API."

"We will review these questions and respond appropriately," the statement added. "We look forward to continuing our long term partnership with the company."

Musk has taken aim at Microsoft over its partnership with the ChatGPT creator, OpenAI, which he cofounded. He said OpenAI had become a "maximum profit company effectively controlled by Microsoft."

In an apparent reference to Microsoft's AI work, Musk tweeted last month: "They trained illegally using Twitter data. Lawsuit time." That was in response to it dropping advertising in response to API fees.

Twitter has also spent millions on developing its own generative AI model, Insider's Kali Hays reported last month.

Insider contacted Twitter for comment. The company responded with an automated message that didn't address the inquiry.