French prosecutors yesterday opened a criminal investigation into Elon Musk and X, escalating a probe into sexual images of minors and other alleged illegal content on Musk’s social network.

Sam Altman, OpenAI’s chief executive, is defending the company against Elon Musk’s allegations that the start-up betrayed its founding mission.

Sutskever said he had prepared a document gathering evidence of Altman’s dishonesty at the request of OpenAI’s board, and confirmed Altman’s conduct included “undermining and pitting executives agains...