The U.S. government has opened an investigation into battery fires in Tesla Model S electric cars.

The National Highway Traffic Administration confirmed that fires occurred in two Model S vehicles in the U.S. after the undercarriage hit metal road debris. The debris then penetrated the batteries which caused a thermal reaction and both fires.

In documents posted on the NHTSA website, the organization said it opened the preliminary evaluation "to examine the potential risks associated with undercarriage strikes" on the vehicles.

As with most probes, the investigation could lead to a recall, but a conclusion may not come for another couple of months.

Company CEO Elon Musk said the company asked the NHTSA to investigate the vehicles to determine if a recall was necessary, but still feels his vehicles are safer than gas-powered cars.

"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," Musk wrote in a blog post this week.

Musk previously said the car wouldn't be recalled but that decision may be out of his hands.

In both incidents the car informed the driver of the damage, and both drivers were able to get out of the vehicle without suffering any injuries, according to NHTSA.

Tesla's batteries are installed under the passenger compartment and are protected by a quarter-inch-thick metal shield, according to the automaker. If the batteries are damaged however it can cause a fire, which has now occurred twice in the U.S. in under two months.

The probe affects over 13,000 vehicles from the 2013 model year that have been sold in the U.S. Tesla has sold over 19,000 vehicles worldwide.

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