Tesla's stock fell as much as 4% after reports that US regulators had sent the company subpoenas as they ramp up investigations into its plans to go private.
Elon Musk, the billionaire chief executive of Tesla, stunned investors with his announcement on Twitter that he had he "secured" funding and would take the company private at $420 a share, valuing it at $72bn.
That tweet may have violated US securities law if he misled investors.
On Wednesday, Fox Business Network senior correspondent Charles Gasparino reported that subpoenas had been issued, saying it showed the "investigation has reached 'formal' stage".
Mr Musk claims Saudi Arabia's sovereign wealth fund has been pushing to take the electric car maker private for the past two years and is still interested.
Shares in Tesla fell back slightly, down 3.1% to $336.63, in New York trading after investment bank Goldman Sachs said it would no longer provide research on the company because it is acting as its adviser.
Mr Musk tweeted on Monday that he was working with Goldman Sachs and private equity firm Silver Lake. However, Reuters says Mr Musk was still negotiating terms with Goldman on Tuesday.
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Tesla says it has formed a special committee of three independent directors to asses any proposal from Mr Musk. It has yet to receive a proposal from the 49-year old.
Tesla and the Securities and Exchange Commission declined to comment on the Fox report.
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