The US Securities and Exchange Commission (SEC) on Thursday urged a federal judge to compel billionaire Elon Musk to testify in an investigation into his $44 billion takeover of social media giant Twitter, now known as X, Reuters reported.

In a filing Thursday in San Francisco federal court, the SEC defended its efforts to compel Musk to testify, saying agency officials were acting within their authority.

Last month, the SEC said it was investigating Musk’s 2022 purchase of shares in Twitter, which Musk later renamed X, as well as his SEC filings and filings related to the takeover of the social media giant.

According to the SEC, Musk declined to be interviewed in September for the investigation.

On November 2, Musk and his lawyers asked a judge to reject the SEC’s request to compel him to testify, saying Musk had already testified twice and that the agency was overstepping its authority.

But the SEC rejected those claims on Thursday, saying agency officials do have the right to subpoena testimony and documents during an investigation.

The SEC also previously said it had received new documents in the investigation since their last interview.

The legal battle is the latest flare-up in a bitter relationship between the US market regulator and the world’s richest man.