Elon Musk, the world's richest man, is under investigation by the U.S. Securities and Exchange Commission over his $44 billion takeover of social media giant Twitter, according to a Thursday court filing in which the agency sought to compel Musk to testify.

The investigation — which escalates a long-running feud between the SEC and Musk — concerns whether Musk broke federal securities laws in 2022 when he bought stock in Twitter, which Musk renamed X, as well as statements and SEC filings he made in relation to the deal.

The SEC said it subpoenaed Musk in May 2023 requiring him to provide testimony at the SEC’s San Francisco office, and that Musk had agreed to appear on Sept. 15. But then two days beforehand Musk raised "several spurious objections" and told the SEC he would not appear, the SEC said. Musk also refused to SEC proposals to conduct the deposition in Texas in October or November.

Among his objections was that the SEC was trying to “harass” him and that his counsel needed time to review potentially relevant material contained in a biography of Musk published last month, the SEC said.