Tesla CEO Elon Musk missed the regulatory deadline for announcing his recent purchases of Twitter stock, raising questions about whether he made more than $150 million by hiding his ownership stake while buying more shares.

A filing this week revealed Musk purchased a 9.2% stake in Twitter, making him the largest shareholder. He also joined the company’s board. A subsequent filing amended his ownership to 9.1%.

Musk could face lawsuits by Twitter investors and penalties from the U.S. Securities and Exchange Commission, an agency Musk has warred with for years, experts say.

A regulation enforced by the SEC requires public disclosure within 10 days of a person’s stock purchases hitting a threshold of 5% of a company’s stock. Musk’s initial SEC disclosure filing was made Monday, but the document itself noted that it was required to be filed within 10 days of March 14, indicating he hit the threshold then.

Musk had continued buying Twitter stock after hitting the threshold, his SEC disclosures show. Once his ownership was revealed, the stock skyrocketed in value.

“The rest of the world was ignorant that he was purchasing the shares,” said David Kass, a finance professor at the University of Maryland’s business school. “Once it became known that he was purchasing those shares, you got that spike.”

Kass estimated that the delayed disclosure delivered an extra $156 million in stock value to Musk. Kass said he based his calculation on Musk having bought 13.1 million shares after the deadline, and the value of Twitter stock going up as it did after Musk made the delayed announcement this week.

“I assume it was someone working for Elon Musk who filled out the form for him … a lawyer or some other finance person,” Kass said. “The person who filled it out presumably should’ve known about this requirement. It’s possible that Elon Musk was unaware of it as an oversight, or ignorant, or of course it’s possible that he knew.”

Musk did not immediately respond to a request for comment sent to Tesla, which recently moved its headquarters to Texas from Palo Alto and makes most of its cars at a factory in Fremont. The company on Thursday was holding a grand opening for its new factory in Austin.

Tulane University law professor Ann Lipton noted that if Musk had disclosed his stake at the required 5% threshold, “he would’ve had to pay more” to buy the shares that drove his ownership in Twitter to 9.1%. However, if Musk had filed the announcement by the deadline, and the stock spiked, he may have declined to continue buying shares at that point. “Then he would have fewer Twitter shares,” Lipton said.

Musk could be hit with lawsuits by investors who sold Twitter stock in the period between the deadline Musk missed and the announcement of his stake in Twitter, Lipton said. Potentially, shareholders “sold their stock at prices that were too cheap because, as we saw, the market went up as soon as people knew Musk was involved,” Lipton said. To win such lawsuits, an investor would probably have to prove Musk deliberately delayed his filing, Lipton added.

Musk, famously combative with the SEC, has exposed himself to possible enforcement action by the agency, Lipton said. The agency has tended to respond to such delayed filings with “slap-on-the-wrist” penalties, but its history with Musk could make this situation different, Lipton added. “For all I know, the SEC will say Elon Musk has given us a lot of trouble,” she said. “Whether the SEC, given his history with them, decides it’s a good reason or a bad reason to get more involved, I don’t know.”

The SEC on Thursday declined to comment on Musk’s delayed filing.

The agency in 2018 charged Musk with securities fraud, alleging his “false and misleading tweets” about taking Tesla private had driven Tesla’s stock price up more than 6% and led to “significant market disruption.” In a settlement, the SEC fined him and Tesla $20 million each, but in 2020 the agency accused him of tweeting out business information without the pre-approval of a company lawyer, in violation of the settlement. A group of Tesla shareholders have alleged in a lawsuit in federal court in San Francisco that the taking-Tesla-private tweets, which Musk has told the court were “entirely truthful,” had cost them billions.

Musk has repeatedly taken to Twitter to publicly mock the SEC. In February, responding on Twitter to a mention of his battles with the agency, Musk tweeted, “I didn’t start the fight, but I will finish it.”