■ The U.S. probes four deaths in Hyundai-Kia cars when air bags failed.

DETROIT — Air bags in some Hyundai and Kia cars failed to inflate in crashes and four people are dead. Now the U.S. government’s road safety agency 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.

In documents posted on its website Saturday, the safety agency says the probe covers 2011 Hyundai Sonata midsize cars and 2012 and 2013 Kia Forte compacts. The agency says it has reports of six front-end crashes with significant damage to the cars. Four people died and six were injured.

The problem has been traced to electrical circuit shorts in air bag control computers made by parts supplier ZF-TRW. The agency now wants to know if other automakers used the same computer.

On Feb. 27, Hyundai recalled nearly 155,000 Sonatas because of air bag failures, which the company blamed on the short circuits. Kia has yet to issue a recall.

In a statement Saturday, Kia said that it has not confirmed any air bag nondeployments in its 2002-2013 Kia Forte models arising from “the potential chip issue.”

But a consumer complaint cited in NHTSA’s investigation documents said Kia was informed of a crash near Oakland in which air bags failed to deploy and a passenger was killed.

In October 2015, the complainant told the NHTSA that a 2012 Forte was involved in a serious front-end crash that occurred in July 2013. A passenger was killed and the driver was injured. According to the complaint, Kia was notified, the air bag computer was tested and it was “found not to be working.”