U.S. auto safety regulators have stepped up a long-running investigation into potentially defective air bag inflators that can cause metal fragments to strike vehicle occupants, saying it required auto supplier ARC Automotive Inc. to answer questions.

The National Highway Traffic Safety Administration (NHTSA) in May demanded the recall of 67 million air bag inflators produced by ARC because they could project metal fragments into vehicle occupants. ARC rejected the request.

NHTSA's special order made public Thursday requires the company to answer written questions under oath and provide documents by June 14.

ARC inflators are installed in vehicles made by General Motors, Chrysler-parent Stellantis, BMW, Hyundai Motor and Kia Corp. vehicles, among other automakers. GM recalled 1.03 million vehicles with ARC air bag inflators in May after a rupture resulted in facial injuries to a driver.

NHTSA demanded that ARC answer whether it contends air bag inflators are expected to occasionally rupture and if it had notified customers of this.

The agency said it wants the company to state the frequency of field ruptures of the subject inflators which it anticipated to occur as random "one-off" anomalies.

ARC, which did not immediately comment Thursday, last month rejected NHTSA's tentative conclusion that a defect exists, saying it was based on just seven field ruptures in the United States.

NHTSA "then asks ARC to prove a negative – that the 67 million inflators in this population are not defective," it said.

NHTSA in 2016 upgraded a probe first opened in July 2015 of more than 8 million air bag inflators made by ARC after a driver was killed in Canada in a 2009 Hyundai Elantra.

The recall demand covers 67 million inflators produced through January 2018. Delphi, acquired by Autoliv, manufactured approximately 11 million of the inflators under a licensing agreement with ARC, which manufactured the remainder.

NHTSA said it has reports of two fatalities – the Canadian death and the August 2021 death of a driver in a 2015 Chevrolet Traverse ruptured in Michigan that had a replacement air bag module produced by ARC.