SpaceX and Tesla chief Elon Musk is under the lens for his tardy submission of a public form that all investors must file when they buy more than five percent of a company’s shares, Wall Street Journal reported on Thursday, citing people familiar with the matter.

Musk started buying Twitter shares from January and secured a five percent stake in the microblogging site by March 14. As per the guidelines of the Securities and Exchange Commission (SEC), the US federal body set up to prevent market manipulation, Musk should have informed the regulator about his stakes within 10 days of getting to the five percent mark. He "missed" the deadline and only made his SEC filing on April 4.

On April 12, Twitter investor Marc Bain Rasella filed a lawsuit against Musk in a New York federal court for not revealing his Twitter stake to the SEC within the deadline. Rasella alleged Musk saved $143 million illegally by not disclosing his Twitter stake.

While Musk hasn’t publicly explained the reason behind his belated disclosure of the Twitter stake, federal regulators, including the SEC and the Federal Trade Commission (FTC), have started investigating the "lag" that allowed the billionaire to buy more shares without alerting other shareholders, said the WSJ report.

Recently, SEC investigators also sought related documents from Twitter, it added.

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Disclosure of a five percent stake in a company by any investor is considered crucial as it is an early sign to shareholders that a significant investor could seek to control or influence the particular company. According to official data, Musk bought Twitter shares worth $513 million between March 24 (10 days after he got to the five percent stake mark) and April 4 (when he disclosed his stake).

According to US law experts, the SEC investigation is unlikely to derail the Twitter deal because the company’s board of directors endorse it. Additionally, the SEC lacks the power to stop mergers or take-private transactions, said the report.

On the contrary, the FTC, which is investigating whether Musk violated a law that requires companies (and people) to report certain large transactions to antitrust-enforcement agencies, can impose fines of up to $43,792 a day, it added.