Federal regulators announced Tuesday that they will investigate what caused three Tesla Model S plug-in electric vehicles to catch fire in six weeks.

Tesla CEO Elon Musk said on the company's blog that Tesla had requested the investigation. The National Highway Transportation Safety Administration did not confirm that, instead saying that the investigation was "prompted by recent incidents in Washington state and Tennessee that resulted in battery fires due to undercarriage strikes with roadway debris."

Musk said Tesla already has upgraded the Model S to improve "ground clearance at highway speeds and would change its warranty policy to cover fire-related damage."

The NHTSA probe comes as shares of Tesla have plunged following the incidents. The electric car maker's stock was at $116 Tuesday compared with about $180 a month ago.

Musk defended the safety of the Model S, noting that gasoline-powered cars present a much likelier chance of catching fire than Tesla vehicles. He said that's because gasoline tanks possess 10 times more combustion energy than Tesla battery packs.

"The far more deadly nature of a gasoline car fire deserves to be re-emphasized," Musk said. "Since the Model S went into production mid-last year, there have been over 400 deaths and 1,200 serious injuries in the United States alone due to gasoline car fires, compared to zero deaths and zero injuries due to Tesla fires anywhere in the world."