Dec 12 (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 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 making a monetary payment or face charges on multiple

counts.

The amount demanded was not in the letter.

Musk has long sparred with the SEC and last year four

lawmakers asked the commission to investigate whether Elon Musk

committed securities fraud by allegedly misleading investors

about the safety of a brain implant developed by Neuralink.

How much traction the SEC would gain in any action against

Musk is unclear.

The billionaire entrepreneur who also heads Tesla

and SpaceX is set to gain extraordinary influence after spending

more than a quarter of a billion dollars to help Donald Trump

win November's presidential election. His companies are expected

to be well insulated from regulation and enforcement measures.

Trump has also appointed Musk to a task force that plans a

sweeping overhaul of the U.S. government.

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 respond to Reuters

requests for comment outside regular business hours.

Musk's tussles with the SEC include the regulator's probe

into his $44 billion takeover of Twitter, now X.

A federal judge in November rejected the SEC's request to

sanction Elon Musk after he failed to appear for court-ordered

testimony in relation to the matter.

The commission also sued Musk in 2018 over his Twitter posts

about taking Tesla private. He settled that lawsuit by paying a

$20 million fine, agreeing to let Tesla lawyers review some

posts in advance and stepping down as Tesla's chairman.