(Bloomberg) -- Elon Musk renewed his longstanding feud with the Securities and Exchange Commission on Thursday by posting a letter that said the agency is investigating his brain-computer interface company, Neuralink Corp.

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The letter that Musk posted on X, which appeared to be from his long-time lawyer Alex Spiro, said the commission this week “reopened” a probe into Neuralink. It also said the SEC is preparing to take action against him related to his investment in Twitter Inc. before he bought the platform in 2022.

The regulator has been examining the timing and disclosure of Musk’s purchase of Twitter stock before he acquired the company for $44 billion. The letter said the agency gave Musk a 48-hour deadline to accept a settlement over the probe or face an enforcement action. The SEC declined to comment.

In the letter, Spiro did not say what the SEC was investigating about Neuralink.

The letter was addressed to outgoing SEC Chair Gary Gensler. In it, Spiro said that the commission recently subpoenaed him, but that he refused to testify. The lawyer demanded to know whether it was Gensler or “the White House” who is directing an “improperly motivated campaign against Mr. Musk.”

Spiro didn’t immediately respond to a request for comment.

Musk has had a testy relationship with the SEC for years, going back to when it sued him for securities fraud in 2018 after he tweeted about taking Tesla Inc. private. He took another dig at the agency on X earlier Thursday.

In the fall of 2023, animal advocacy group the Physicians Committee for Responsible Medicine and four US lawmakers asked the SEC to investigate Musk and Neuralink for possible securities fraud over what the group said were misleading claims by the company.

--With assistance from Kurt Wagner and Sarah McBride.

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