(Reuters) - The U.S. Securities and Exchange Commission (SEC) this week has reopened an investigation into Elon Musk's brain-chip startup Neuralink, according to a letter shared by Musk on Thursday on social media platform X.

The Dec. 12 letter from Musk's lawyer Alex Spiro and addressed to outgoing SEC Chair Gary Gensler also noted that the commission had issued a settlement demand and that Musk had been given 48 hours to accept a a monetary payment or face charges on multiple counts.

Spiro wrote in the letter that he and Musk would not be intimidated by the SEC and that they reserved their legal rights.

The SEC and Neuralink did not immediately did not respond to Reuters requests for comment outside regular business hours.