In September, Hyundai and Kia issued a recall of 3.4 million of its vehicles in the United States with an ominous warning: The vehicles should be parked outdoors and away from buildings because they risked catching fire, whether the engines were on or off.

Nearly three months ago, Hyundai and Kia unveiled software that was designed to thwart an epidemic of thefts of their vehicles, caused by a security flaw that was exposed on TikTok and other social media sites.

U.S. auto safety regulators have stepped up a series of investigations into multiple engine fires that have plagued Hyundai and Kia vehicles for more than six years.