U.S. regulators opened an investigation into Elon Musk’s tardy disclosure of his substantial stake in Twitter (NYSE:) last month as the delay allowed Tesla (NASDAQ:) CEO to purchase additional shares of the social media company without informing other shareholders about his stake, according to the Wall Street Journal.

The WSJ reported that the Securities and Exchange Commission (SEC) is investigating Musk’s belated filing of a public form that investors are required to send when purchasing a stake in a company larger than 5%.

According to the report, Musk declared his stake in Tesla on April 4, 10 days after his purchase exceeded the threshold point for submission. Tesla’s boss did not say why he did not declare his stake in due time.

According to Dr. Daniel Taylor of the University of Pennsylvania’s accounting department, Musk has saved $143 million because he did not declare that his share had increased beyond 5%. The market could have risen higher if he disclosed his increasing stake.

An investor who increases their stake beyond 5% in a company must file an SEC form revealing the new size of their stake within 10 days. Musk’s stake in Twitter was 5% at March 14th, so he had to file a form with the SEC by March 24.

“The case is easy. It’s straightforward. But whether they’re going to pick that battle with Elon is another question,” said Dr. Taylor.

Pre-market on Thursday saw Twitter’s stock fall by 2% to $45.15, as reported at 06:25 EST.

By Senad Karaahmetovic