Tesla has said the US Department of Justice was investigating chief executive Elon Musk's public statements in August that he was considering taking the electric car maker private.

This marks the latest and biggest threat to Musk's leadership. 

The US Justice Department asked Tesla for documents about Musk's announcement, the company said, describing it as a "voluntary request." 

Tesla said it was co-operating and that the matter "should be quickly resolved." 

The probe by the Justice Department, which can press criminal charges, comes on top of a civil probe by the Securities and Exchange Commission and shareholder lawsuits. 

Musk had surprised investors on August 7 with his plan to take Tesla private, tweeting that he had "funding secured" for a deal that would have valued the company at $72 billion. 

In a separate tweet, he wrote: "Investor support is confirmed." 

Tesla's shares rose after his tweet, but investors in the company's bonds and convertible debt were skeptical that the tens of billions of dollars needed for the buyout would materialise. 

After two weeks of uncertainty, particularly over funding, Musk abruptly abandoned the plan on August 24, saying it would be even more time-consuming and distracting than anticipated, and that "most of Tesla’s existing shareholders believe we are better off as a public company." 

Tesla's stock, which has lost about 25% since its gains after Musk first tweeted about going private, fell 3.5%to $284.50 yesterday. 

The SEC has already opened an inquiry into Musk's tweets, according to one person with direct knowledge of the matter, and short-seller Citron Research has sued Musk over the truthfulness of his tweet about secured funding. 

The SEC has not commented on the case. 

The SEC has in the past month sent subpoenas also to investment bank Goldman Sachs and buyout firm Silver lake as they were hired by Tesla to explore the going-private deal, the New York Times said. 

Tesla biggest institutional investor Baillie Gifford said last week it was questioned by the SEC about Musk's plans to take the electric carmaker private. 

An executive with the asset manager said the chief executive needed help running the company. 

To charge Musk with a crime, the Justice Department would need to show that he intended to manipulate Tesla’s stock price, said Henning, now a law professor at Wayne State University.

For a civil enforcement action, the SEC would only need to show that Musk acted negligently, which is easier to prove, he said. The SEC could subject Musk to civil sanctions such as fines, relinquishing improper profits and a ban on running public companies.

Musk's recent behavior, which included smoking marijuana during a video interview, has raised concerns about his leadership, with several Wall Street analysts and some investors urging Tesla to appoint a strong second-in-command. 

He has been under intense pressure to prove he can deliver consistent production numbers for the Model 3 sedan, which is crucial to Tesla's plan to become a mass-market car maker.