Nearly a decade after the first recalls were issued for cars fitted with defective Takata-made inflators, a seven-year federal investigation into another airbag component manufacturer is picking up steam.

The National Highway Traffic Safety Administration is searching for information about inflators produced by Tennessee-based ARC Automotive Inc. As reported by The Wall Street Journal, NHTSA has sent letters to several vehicle manufacturers enquiring about the potentially defective inflators, which could be fitted to tens of millions of cars.

According to NHTSA, the probe relates to 51 million ARC inflators used for both passenger and driver-side airbags and installed between model years 2002 and 2017. While NHTSA doesn’t specify how many cars may be affected, court documents reveal that on U.S. roads, these components have fired six times, leaving two dead and four injured as a result of airbag shrapnel that exploded in a crash.

Read: Honda Tried Contacting Accord Driver Who Died From Takata Airbag Over 300 Times In 11 Years

General Motors, Ford, BMW, and Volkswagen have all issued comparatively small recalls (relating to some 6,400 vehicles) in regard to the investigation, and these manufacturers were among those contacted by this week’s NHTSA letter.

Speaking to WSJ, GM confirmed that it was working with the federal safety regulator and has retained the expertise of a third-party engineering firm with experience in airbag inflator performance. The American automaker also claimed that it had no evidence of a design defect or systematic malfunction to support an expansion of the previous recalls.

The investigation bears similarity to the one launched in 2014 surrounding Takata. which led to the recall of roughly 42 million vehicles. In November — after more fatalities — NHTSA warned that 276,000 U.S. drivers were still at risk, having not fixed the defective airbags.