The US government's car safety watchdog has opened an investigation into battery fires in Tesla Model S electric cars.
The National Highway Traffic Administration said fires broke out in two of the cars in the US after the underneath hit metal road debris which then pierced the batteries and caused a thermal reaction and fires. In each case, the car warned the driver and they escaped unhurt.
The probe affects more than 13,000 cars from the 2013 model year that were sold in the US. Tesla has sold about 19,000 of the cars worldwide.
The batteries are beneath the passenger compartment and protected by a quarter-inch-thick metal shield. Experts say that if theys are damaged, they can cause arcing and sparks and touch off a fire.
The NHTSA said it opened the preliminary evaluation "to examine the potential risks associated with undercarriage strikes" on the Tesla cars. The investigation could lead to a recall, but a decision likely is months away.
Tesla chief executive Elon Musk said that the company asked the government to investigate, even though its cars catch fire at a far lower rate than petrol-powered vehicles.
"While we think it is highly unlikely, if something is discovered that would result in a material improvement in occupant fire safety, we will immediately apply that change to new cars and offer it as a free retrofit to all existing cars," he said.
He also wrote that Tesla has done an over-the-air software update to give the car more ground clearance at road speeds.
He said that the software change was made to cut the chances of underbody damage. "The theoretical probability of a fire injury is already vanishingly small, and the actual number to date is zero."