Air bags in some Hyundai and Kia cars failed to inflate in crashes and four people are dead and officials wants to know why. The National Highway Traffic Safety Administration says it’s investigating problems that affect an estimated 425,000 cars made by the Korean automakers. The agency also is looking into whether the same problem could happen in vehicles made by other companies. The agency says the probe covers 2011 Hyundai Sonatas and 2012 and 2013 Kia Fortes. The agency has reports of six front-end crashes with significant damage. Six other people were injured. The problem has been traced to electrical circuit shorts in air bag control computers made by parts supplier ZF-TRW. NHTSA will investigate whether other automakers used the same computer.