The complaint filed by the Securities and Exchange Commission came after a last-minute decision by Mr. Musk and his lawyers to fight the case rather than settle the charges.

The filing by the SEC in federal court in Manhattan threatens to deal a severe blow to the Palo Alto, Calif., electric car maker. Its brand and Mr. Musk are closely intertwined, and analysts have said the company’s roughly $50 billion market value is driven by Wall Street’s appreciation for Mr. Musk’s vision and skill as an innovator.

Tesla wasn’t named in the suit as a defendant, but the SEC is seeking to bar Mr. Musk, Tesla’s largest shareholder and its top executive, from serving as an officer or director of any U.S. public company. Tesla shares, which have been under intense pressure amid questions about the firm’s financial strength and Mr. Musk’s behavior, tumbled 9.9% to $277 in after-hours trading Thursday on Nasdaq.

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The SEC said that contrary to the statements he made in several Twitter messages on Aug. 7, Mr. Musk “knew that he had never discussed a going-private transaction at $420 per share with any potential funding source.” The agency said the statements and omissions of fact caused disruption to the market for Tesla shares—which rose more than 10% the day of the tweets—and harm to investors.

“It’s an easy case,” said Charles Elson, director of the John L. Weinberg Center for Corporate Governance at the University of Delaware. “He said in the tweet he had financing, and apparently he didn’t. … It’s about as straightforward as you can get.”

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