U.S. safety regulators are investigating fires in Tesla Model S electric cars.
The National Highway Traffic Administration said fires broke out in two of the cars' batteries after the undercarriage hit road debris. The debris damaged the batteries and caused a thermal reaction and fires. No one was hurt in the fires.
The probe affects more than 13,000 cars sold in the U.S. The agency says wants to examine risks from the undercarriage getting hit.
"The National Highway Traffic Safety Administration is deeply committed to safeguarding the driving public," the agency said in a statement to CNBC. "The agency has opened a formal investigation to determine if a safety defect exists in certain Tesla Model S vehicles. The agency's investigation was prompted by recent incidents in Washington State and Tennessee that resulted in battery fires due to undercarriage strikes with roadway debris."
(Read more: Yet another Model Sfire puts heat on Tesla)
Tesla's batteries are located beneath the passenger compartment and protected by a quarter-inch-thick metal shield.
Tesla says it asked the government to investigate even though its cars catch fire at a far lower rate than gas-powered vehicles.
The investigation could lead to a recall, but no decision has been made.
(Read more: What went wrong withTesla? Jim Cramer explains)
In a post to the Tesla blog on Monday, Chairman Elon Musk stated: "There are now substantially more than the 19,000 Model S vehicles on the road that were reported in our Q3 shareholder letter for an average of one fire per at least 6,333 cars, compared to the rate for gasoline vehicles of one fire per 1,350 cars. By this metric, you are more than four and a half times more likely to experience a fire in a gasoline car than a Model S! Considering the odds in the absolute, you are more likely to be struck by lightning in your lifetime than experience even a non-injurious fire in a Tesla."
On Tuesday, Musk tweeted from his Twitter feed: @elonmusk: Why does a Tesla fire w no injury get more media headlines than 100,000 gas car fires that kill 100s of people per year?
—By The Associated Press. CNBC contributed to this report.