WASHINGTON — Federal regulators are investigating Elon Musk’s late disclosure last month of his sizeable stake in Twitter, according to people familiar with the matter, a lag that allowed him to buy more stock without alerting other shareholders to his ownership.

The Securities and Exchange Commission is probing Musk’s tardy submission of a public form that investors must file when they buy more than 5% of a company’s shares, the people said. The disclosure functions as an early sign to shareholders and companies that a significant investor could seek to control or influence a company.