Elon Musk, the world's richest man and a top adviser to US President Donald Trump, was issued a summons in connection with the Securities and Exchange Commission's lawsuit against him, a court filing on Thursday showed.

A process server gave the civil summons and other documents on March 14 to a security guard at the Brownsville, Texas, headquarters of SpaceX, the space technology company of which Musk is CEO, the filing said. An answer is due on April 4, according to the docket.

ALSO READ: US SEC sues Elon Musk over late disclosure of Twitter stake

The SEC in January accused Musk of waiting too long to disclose in 2022 that he had amassed a large stake in Twitter, the social media company he later bought and renamed X.

The regulator said Musk violated federal securities law by waiting 11 days too long to disclose his initial purchase of 5 percent of Twitter's common shares.

READ MORE: US judge rejects SEC bid to sanction Elon Musk

An SEC rule requires investors to disclose within 10 calendar days -- or by March 24, 2022, in Musk's case -- when they cross a 5 percent ownership threshold.

Musk and his lawyer didn't immediately respond to requests for comment. A spokesperson for the SEC declined to comment.