The U.S. Securities and Exchange Commission filed a lawsuit Tuesday accusing X owner Elon Musk of violating federal securities law by failing to disclose his stake in Twitter in early 2022.

According to the SEC, Mr. Musk began buying Twitter stock with a 5% stake in the social media platform by March of that year. Under federal securities laws, the SEC says, he was required to disclose that information.

The agency says Mr. Musk waited a month to report his stake, letting him underpay for shares in Twitter and saving the billionaire upward of $150 million.

Tuesday’s suit is a major escalation in the SEC’s probe into Mr. Musk. Shortly after the Twitter purchase, the SEC started investigating if any securities laws had been broken by him or his allies.

Over the past two years, the SEC has tried unsuccessfully to get Mr. Musk to provide what it considers to be adequate testimony on the Twitter purchase. Last year, the commission sought a court order to force him to testify again before investigators. Mr. Musk and his lawyers have accused the SEC of harassment.

Mr. Musk’s main lawyer, Alex Spiro, called this week’s suit a sham.

“As the SEC retreats and leaves office — the SEC’s multiyear campaign of harassment against Mr. Musk culminated in the filing of a single-count ticky-tack complaint against Mr. Musk under Section 12(d) for an alleged administrative failure to file a single form,” Mr. Spiro wrote in a statement. “An offense that, even if proven, carries a nominal penalty.”

The suit comes as Donald Trump prepares to retake the White House on Monday and shake up U.S. regulatory oversight. SEC Chairman Gary Gensler will step down from his position that day, making the future of the Musk lawsuit uncertain.

• Vaughn Cockayne can be reached at vcockayne@washingtontimes.com.