Kia Motors Corp. said on Friday it is recalling more than 507,000 vehicles in the United States because an electronic glitch may prevent air bags from deploying in the event of a crash.

The recall follows an announcement in March by the National Highway Traffic Safety Administration (NHTSA) that it was investigating why some air bags had failed to deploy in Kia vehicles and its affiliate Hyundai Motor Corp after crashes in which four people were killed and another six injured involving the two automakers' vehicles.

In total, the two Korean automakers have now recalled nearly 1.1 million U.S. vehicles to address the issue. NHTSA said in March it was aware of six serious crashes in which air bags failed to deploy in frontal crashes, including four in 2011 model Hyundai Sonatas and two in 2012 and 2013 Kia Forte vehicles. The crash of the 2013 Forte occurred in Canada.

Kia's recall issued on Friday covers 2010-2013 Kia Fortes, 2011-2013 Kia Optimas and 2011-2012 Kia Optima Hybrid and Sedona vehicles. The company said the air bag control unit may short circuit because they may be susceptible to electrical overstress, preventing the frontal air bags and seat belt pretensioners, which pull the driver and front seat passenger firmly back into their seats, from deploying.

The company said it does not yet have a fix, but is working with its supplier on the issue.

Hyundai in February issued a recall for 154,000 U.S. Sonatas after non-deployment reports were linked to electrical overstress in the air bag control unit. In April, Hyundai recalled an additional 425,000 U.S. vehicles to address the same issue.

Hyundai said in March it was aware of reports of two deaths in its vehicles, which occurred in head-on collisions at extremely high rates of speed.

NHTSA said the air bag control module under investigation was built by ZF Friedrichshafen AG, a German auto supplier. The company said in March it was working with NHTSA.

The safety agency also said that electrical overstress appeared to be the root cause in a 2016 recall by Fiat Chrysler Automobiles NV of 1.4 million U.S. vehicles for air bag non-deployments in significant frontal crashes.