Twitter CEO Elon Musk threatened to take legal action against Microsoft on Wednesday after accusing the company of “illegally using Twitter data.” Musk’s comment came in response to a tweet saying that Microsoft had dropped Twitter from its advertising platform “as they refuse to pay” new fees to access Twitter’s API. “They trained illegally using Twitter data,” Musk wrote, referring to Microsoft. “Lawsuit time.” The billionaire didn’t go into specifics, but his allegation could refer to OpenAI using the social media site’s data to train its AI-powered software like ChatGPT. OpenAI recently received major investment from Microsoft, which is currently looking to integrate AI into Bing and other tools.