The US Securities and Exchange Commission has sued Tesla Chief Executive Elon Musk, accusing him of securities fraud for making a series of "false and misleading" tweets about potentially taking the electric car company private.
In a complaint filed in Manhattan federal court, the SEC said Mr Musk "knew or was reckless in not knowing" that he was misleading investors on 7 August by telling his more than 22 million Twitter followers that he might take Tesla private at $420 per share, and that there was "funding secured."
The complaint also faults subsequent tweets in which Mr Musk said "investor support is confirmed," and that a "special purpose fund" might be created for investors who stick with the Palo Alto, California-based company.
Tesla was not immediately available for comment. Its shares fell 5.7% in after-hours trading.
Mr Musk has issued a statement saying that he rejects the charges.
"This unjustified action by the SEC leaves me deeply saddened and disappointed," he said.
"I have always taken action in the best interests of truth, transparency and investors ... and the facts will show I never compromised this in any way."
The lawsuit makes Mr Musk one of the highest-profile executives to be accused by the SEC of securities fraud.
It also seeks to bar him from running public companies, which would include Tesla, as well as a civil fine.
The SEC does not have criminal enforcement power.
Mr Musk has long used Twitter to criticise short-sellers betting against his company, and several investor lawsuits have been filed against him and Tesla over the tweets.
On 24 August, after news of the SEC probe had become known, Mr Musk blogged that Tesla would remain public, citing investor resistance.