Shares of Tesla (TSLA - Get Report) slid after reports that the government is embarking on an investigation of fires that have occurred in its electric vehicles.

The National Highway Traffic Safety Administration wrote a letter to Tesla last week, saying it's investigating fires in Model S and Model X cars built from 2012 to 2019 and sold in the U.S.

NHTSA said it's looking at recent Tesla software updates, "which have been alleged to be issued by Tesla in response to the alarming number of car fires that have occurred worldwide."

On several occasions, Tesla batteries caught fire without apparent cause -- both in parked and moving vehicles, various media have reported. NHTSA is asking Tesla for any information it has about other fires that the media haven't reported. 

A May fire in a Model S car in Hong Kong prompted Tesla to update Model S and Model X software, limiting battery charging to 80% of capacity, the Los Angeles Times reports. The company said it was acting to "protect the battery and improve its longevity," but didn't elaborate. 

Many Tesla owners protested that the move cut their cars' range, according to the paper. It said the battery limitation move led to a class-action lawsuit and numerous customer complaints. 

Tesla shares traded at $313.07, down 0.6%.

The stock soared 29% in two days last week after news of stronger-than-expected earnings for the company in the third quarter. But Tesla shares are down 5% since Oct. 25 amid questions about earnings quality. 

After the profit report, Morningstar analyst David Whitson raised his fair value estimate to $326 from $231. 

But "we remain concerned about Tesla's debt load, so if free cash flow becomes insufficient to service debt, we may raise our weighted average cost of capital." he wrote.

"All else constant, that move would lower our fair-value estimate to as low as $206. Tesla is a volatile name, and fair-value-estimate changes may be frequent as its story changes."

Whitson rates Tesla three stars, or fairly valued.

The author owns no Tesla shares.