Federal safety regulators opened a probe into a series of fires in Tesla Motors Inc. electric cars following crashes.
The National Highway Traffic Safety Administration said Tuesday that it would investigate why two of the expensive Tesla Model S sporty hatchbacks have caught fire this year, following accidents in which the battery casing that serves as the undercarriage of the car cracked open.
A third fire, which took place in Mexico, won't be part of the investigation because it is outside the agency's jurisdiction.
NHTSA noted that in the two U.S. fires, one in Seattle and the other in Tennessee, the battery monitoring system in the cars warned the drivers, allowing them to pull over and safely exit the vehicles before the battery emitted smoke and fire.
The fires have become an issue for Tesla, contributing to about a 35 percent drop in the value of its stock since Sept. 30, when shares reached a high of $193.37.
Tesla chief executive Elon Musk said the automaker requested the investigation to quell fears that electric cars are more prone to fires than other vehicles. But the NHTSA disputed that, saying its decision was an independent process.
"In regards to Tesla, the agency notified the automaker of its plans to open a formal investigation and requested their cooperation," the agency said, "which is standard agency practice for all investigations."
Tesla is already taking action, changing its warranty to cover damage due to a fire, even if due to driver error. "Either our belief in the safety of our car is correct, and this is a minor cost," Mr. Musk said, "or we are wrong, in which case the right thing is for Tesla to bear the cost rather than the car buyer."
The automaker believes that the warranty change, which doesn't apply if an owner purposefully sets the car on fire, will reassure buyers.
Mr. Musk also defended the automaker's vehicles, writing that they have a lower fire risk than gasoline vehicles. "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," he wrote.
But others said the fires were cause for concern, considering that other electric cars were not set ablaze by crashes. "While only three Tesla fires have occurred, that's three more than I'm aware of for the Nissan Leaf, which has sold in greater numbers while being on the market longer," said Karl Brauer, senior analyst at Kelley Blue Book, the auto information company.
The NHTSA probe is the latest setback for the Palo Alto, Calif., car company, which until last month looked to be accelerating as fast as its high-powered Model S. Tesla was once among the hottest stocks on Wall Street. Its shares soared from $33.87 on Dec. 31 to its all-time high of $193.37 on Sept. 30. That growth was powered by brisk Model S sales, a glowing review from Consumer Reports, top results in federal crash tests, and more than $100 million in sales of regulatory environmental credits to other automakers. The company is on track to sell about 21,000 of its cars, which start at more than $70,000.
Investor concern about the fires is behind much of the recent sell-off. But other issues have also contributed to the decline, including a battery supply bottleneck, heavy research and development spending, and declining sales of lucrative environmental credits.
Tesla shares rose Tuesday after the federal safety probe was announced.