The NHTSA opened the Office of Defects Investigation into the air bag issue related to crashes in March.
The recall affects 507,587 vehicles, including 2010-13 Kia Forte, Forte Koup sedans; 2011-13 Kia Optima sedans; and 2011-12 Kia Optima Hybrid sedans and Sedona minivans, a National Highway Traffic Safety Administration release states.
It was added that a few injuries caused by the issues have been reported.More news: Oops! 'Fox and Friends' host calls Trump a 'dictator'
The problem also impacted Hyundai, which is affiliated with Kia. In these crashes, four people were killed and another six were injured. The safety agency said at the time it had reports of six front-end crashes with significant damage to the cars. KIA says it is working with its supplier to fix the issue. Kia said it would notify owners by July 27, while Hyundai said its notification runs from April 20 to June 15. ZF-TRW also said the parts are made to auto manufacturers' specifications.
ZF said on Friday that it has worked with Kia and "continues to cooperate and support NHTSA and its customers in the investigation".
The 2013 Forte crash took place in Canada.More news: Twitter Ablaze as Abramovich Refuses to Sell Chelsea to UK's Richest Man
In May of previous year, NHTSA opened an investigation into whether Hyundai and Kia moved quickly enough to recall more than 1.6 million cars and SUVs because the engines can stall, increasing the risk of a crash.
In March, NHTSA said it was investigating if other automakers used similar air back control units and if they could pose a risk.More news: 'The Band's Visit' and 'Harry Potter' win big at Tonys