Following the publication of documents from the National Highway Traffic Safety Administration (NHTSA), it has been revealed that as many as four million Kia vehicles could be equipped with an airbag inflator that the organization says poses a risk to occupants.

In May, the regulator said that a total of 67 million vehicles in the U.S. may be equipped with airbags that contain defective inflators. It alleged that these parts could actually explode and shoot shrapnel into the cabin, injuring occupants.

Although NHTSA began its investigation in 2015 following two reports of injuries, the manufacturer, Tennessee’s ARC, disagrees with its findings. It claims that its own investigation has not found a defect that warrants a recall of 67 million vehicles.

More: U.S. Wants To Recall 67 Million Airbags, Supplier Disagrees

 As Many As 4 Million Kias Could Be Equipped With Potentially Dangerous Airbag Inflators

The newly released documents show that one of the instances of injury related to this issue came from a 2004 Kia Optima, which occurred in 2014, reports the Wall Street Journal. In addition, it is the first time that an official NHTSA document has identified vehicles potentially affected by this defect.

The four million vehicles the brand may have to recall were from two model lines and were produced over a 16-year period, starting in 2000. Still, no exhaustive list of vehicles that are impacted by this potential safety issue has been released.

However, Kia is far from the only automaker implicated in the problem. If NHTSA gets its way, 67 million vehicles will be included, which would make this one of the largest recalls in U.S. history.

In addition to Kia, we know that some GM models are impacted. That’s because the automaker initiated a voluntary recall of 994,763 vehicles with ARC airbag inflators, including Buick Enclaves, Chevrolet Traverses, and GMC Acadias from the 2014-2017 model years.

 As Many As 4 Million Kias Could Be Equipped With Potentially Dangerous Airbag Inflators
2011 Kia Optima