FILE - In this April 26, 2017, file photo, the logo of the Hyundai Motor Co. is displayed at the automaker's showroom in Seoul, South Korea. 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. (AP Photo/Lee Jin-man, File)

The US National Highway Traffic Safety Administration has opened an investigation into a series of deadly crashes in which airbags in Hyundai and Kia cars failed to inflate.

The administration's Office of Defects Investigation (ODI) "is currently aware of six crashes with significant collision-related damage events involving Hyundai and Kia models where airbags failed to deploy in frontal crashes," which four people died and six were hurt, its website said.

The "failure of the airbag control unit (ACU) may prevent the frontal airbags from deploying in the event of a crash," it said.

Hyundai filed a report on the issue at the end of February that led to a recall, but the firm said it had not yet "identified a remedy."

The recall involved 155,000 Hyundai Sonata vehicles produced between December 2009 and September 2010 at their plant in the southern state of Alabama.

"Hyundai is aware of three rare and unique accidents where airbag control circuitry was confirmed to be damaged and a fourth accident is under investigation," the South Korean automaker said.

"We are actively investigating what exactly causes the airbag control unit to become damaged in these specific types of accidents," it said.

Kia is partially owned by Hyundai, and together they form the world's fifth largest automaking group. A Kia spokesperson was unavailable for comment.

The ODI investigation, which was opened Friday, will evaluate the scope of the recall, will determine if Kia and other manufacturers are using the same or similar airbag control unit as Hyundai, and review factors that may be causing the failures, it said.

An estimated 425,000 cars may be affected by the issue.