US regulators are investigating Elon Musk's delayed disclosure of his sizable stake in Twitter Inc last month, the Wall Street Journal reports, citing people familiar with the matter.
Musk disclosed a 9.2 percent stake in Twitter to the US Securities and Exchange Commission (SEC) on April 4, a delay of at least 10 days since surpassing the 5 percent threshold for revealing a shareholding, the report said.
An investor who crosses a 5 percent stake must file a form with the SEC within 10 days. It serves as an early sign to stakeholders that a big investor could seek to control the company.
The SEC declined to comment on the report and the Tesla boss did not immediately respond to a Reuters request for comment.
Apart from the delay, Musk's April 4 filing also characterised his stake as passive, meaning he did not plan to take over Twitter or influence its management or business.
The next day, however, he was offered a position on Twitter's board, and a couple of weeks later, the world's richest man had clinched a US$44 billion deal to buy the social media giant.
Musk, known for his candid Twitter posts, has a long history of skirmishes with the SEC.
Most recently, a US judge slammed him for trying to escape a settlement with the SEC requiring oversight of his Tesla tweets.
In April, the Information reported that the Federal Trade Commission is investigating whether Musk violated a law that requires companies and people to report certain large transactions to antitrust-enforcement agencies.