DETROIT — The U.S. government’s auto safety watchdog has closed an investigation into Tesla electric car battery fires after the company said it will install more shields beneath the cars.

The shields, an aluminum bar, a titanium plate and another piece of aluminum, will supplement a quarter-inch-thick aluminum plate on the Model S, the only model that Tesla now sells. They’re designed to stop road debris from penetrating the car’s battery pack.

The National Highway Traffic Safety Administration began investigating the Model S last year after two battery fires that were caused by road debris. In each case, one near Nashville, Tenn., and another near Seattle, debris punctured the aluminum shield and the battery, touching off fires. Drivers were able to safely pull off the road and escape without injury, but the cars were destroyed.

Tesla CEO Elon Musk said Friday that the company will retrofit Model S cars sold in the U.S. with the new shields, at no cost to owners. At the end of February, Tesla had sold about 22,000 of the cars in the U.S., according to Autodata Corp. The additional shields will be provided upon owner request or as part of normally scheduled service.

The move is not a recall, a Tesla spokeswoman said.

Earlier, Tesla issued a software update that raised the Model S ride height to help deal with the issue.

Musk said the moves bring the risk of debris striking the battery “down to virtually zero” to give Model S owners complete peace of mind.

The new shields, he said, are not needed for a high level of safety, but it’s valuable to minimize inconvenience to owners and address what he said were misperceptions about electric car safety.