Twitter on Thursday accused Microsoft of breaking the social network's rules for developers who access the platform's data, according to a copy of a letter seen by AFP.
"Microsoft may have been in violation of multiple provisions of the Agreement for an extended period of time," read the letter signed by Musk attorney Alex Spiro and sent to Microsoft chief executive Satya Nadella.
Microsoft stopped accessing Twitter data in April, opting not to pay fees Musk demanded developers pay for APIs (application programming interfaces) that engage with the platform, according to the letter.
Twitter called on Microsoft to identify all Twitter content that has been in its control during the past two years; how it is stored and what has been done with it, according to the letter sent to Nadella.
Microsoft confirmed receiving a letter from a law firm representing Twitter with some questions about its previous use of the free Twitter API.
"We will review these questions and respond appropriately," a Microsoft spokesperson said in response to an AFP inquiry.
"We look forward to continuing our long-term partnership with the company."
Twitter said in the letter it is looking into whether Microsoft exceeded the "reasonable request volume" in what could constitute "abusive usage." Twitter wants the information by June 7, the letter stated.
The demand comes as Elon Musk looks to generate revenue by getting developers to pay for Twitter platform access that had been free prior to the billionaire taking over.
Musk is also out to counter Microsoft and Google with his recently-established X.AI artificial intelligence corporation based in the US state of Nevada, according to business documents.
Musk last month fired off a Tweet accusing Microsoft of illegally using Twitter data to train artificial intelligence, writing "lawsuit time."
Big tech companies like Google, Meta and Microsoft have spent years working on AI systems previously known as machine learning or big data — to help with translations, search and targeted advertising.
Microsoft is investing billions of dollars in ChatGPT creator OpenAI and has put its technology to work in its Bing internet search service.
Since taking over Twitter in late October, Musk has repeatedly courted controversy, sacking most of its staff, readmitting far-right figures to the platform, suspending journalists and charging for previously free services.