DETROIT (AP) — Nearly four years ago, the U.S. government’s highway safety agency began investigating air bag inflators made by ARC Automotive of Tennessee when two people were hit by flying shrapnel after crashes.

The National Highway Traffic Safety Administration estimated that 8 million Fiat Chrysler, Hyundai, Kia and General Motors vehicles in the U.S. use the company’s inflators. The investigation became more urgent in 2016 after a Canadian woman driving a Hyundai was killed by shrapnel from an ARC inflator.

But public records posted by the agency show little progress on the probe, which began in July of 2015 and remains unresolved.

Now another person has been hurt by an exploding ARC inflator, this time in a General Motors vehicle. Safety advocates say the slow investigation is a symptom of an agency that has done little to regulate the auto industry.

“That’s really unacceptable. NHTSA should have gotten on top of it sooner,” said Rosemary Shahan, president of California-based Consumers for Auto Reliability and Safety. “It’s just really painfully obvious that it’s a (safety) defect.”

The agency said the investigation remains open “as we continue to review information with ARC and the vehicle manufacturers that may have had ARC inflators installed.”

A spokeswoman did not respond to questions asking about the length of the probe. ARC’s communications office did not return multiple telephone and email messages seeking comment Tuesday and Wednesday.