U.S. securities regulators sued Elon Musk in federal court in Washington on Tuesday in an enforcement action arising from his $44 billion purchase of Twitter, now called X.

The lawsuit against Musk, who has become a close adviser to President-elect Donald Trump, is likely to be one of the more contentious final acts of the Securities and Exchange Commission under Gary Gensler, its departing chair. It could also be undercut in just a few days, when Trump appoints new leadership to take charge of the regulator.

The SEC contends that in buying Twitter in 2022, Musk violated securities laws by amassing a large stock position in the social media company without filing the proper notification. The complaint said he had waited 11 days before filing the required disclosure with the SEC.

The regulatory filings are required so investors in the marketplace can monitor the moves of large investors and potential takeover bids.

Because Musk did not disclose his position, he was able to continue buying Twitter stock at an artificially low price, the SEC said in its lawsuit. The move “allowed him to underpay by at least $150 million” for the additional shares before he belatedly disclosed his stake, the lawsuit continued.

Over the past few weeks, Musk had taunted the SEC in posts on X about the potential for filing a lawsuit. In December, he shared a letter that his lawyer, Alex Spiro, had sent to the agency, rejecting a settlement offer in the case.

On Tuesday, Spiro denounced the SEC’s latest filing.

“Today’s action is an admission by the SEC that they cannot bring an actual case, because Mr. Musk has done nothing wrong and everyone sees this sham for what it is,” Spiro said in a statement. The agency had waged a “multiyear campaign of harassment” against Musk but filed “a single-count ticky-tack complaint,” Spiro added.

This is the third time the SEC has gone to court with Musk. The first lawsuit, during Trump’s first term in office, arose from inappropriate market-moving posts on social media in which Musk mused about taking his electric car company, Tesla, private.

Before filing the lawsuit Tuesday, the SEC had also sought to force Musk to comply with a subpoena seeking to take his deposition.

With Gensler stepping down with the inauguration of Trump on Monday, it is unclear whether incoming regulators will pursue the litigation. Trump has said he intends to nominate Paul Atkins, a former SEC commissioner and pro-business conservative, to succeed Gensler.

Daniel Richman, a professor at Columbia Law School who specializes in criminal law, said the lawsuit appeared to be part of a pattern of matters being filed by Biden administration appointees “on their way out.”

It will be up to the new administration and Trump’s appointees to decide whether to “back off and withdraw” cases like the one against Musk, he said.