DETROIT (AP) — A class action lawsuit is accusing three automakers and a parts manufacturer of knowingly selling vehicles containing air bag inflators that are at risk of exploding. Two deaths and at least four injuries have been linked to such explosions.

The federal lawsuit, filed Tuesday in San Francisco, names ARC Automotive Inc. of Knoxville, Tennessee, which made the inflators and sold them to air bag manufacturers. The air bag makers, in turn, sold them to General Motors, Ford and Volkswagen, which are named in the lawsuit, too.

The five plaintiffs are the owners of vehicles with ARC inflators who contend the defective air bag parts were not disclosed when they made their purchases.

The U.S. National Highway Traffic Safety Administration, which has been investigating ARC inflators for nearly seven years without a resolution, estimates that there are 51 million on U.S. roads. That's somewhere between 10% and 20% of all passenger vehicles.

Yet most drivers have no conclusive way to determine whether their vehicle contains an ARC inflator. Even if they were to tear apart the steering wheel assembly, the internal parts might bear the markings only of the automaker or the air bag manufacturer, not the inflator maker.

"You could have a ticking time bomb in your lap and you've got no way of knowing," said Frank Melton, a Florida lawyer who is among those filing the new lawsuit.

One of the deaths was a mother of 10 who was killed in what appeared to be an otherwise minor crash in Michigan's Upper Peninsula last summer. Police reports show that a metal inflator fragment hit her neck in a crash involving a 2015 Chevrolet Traverse SUV.

In a statement Tuesday, GM said it hadn't had a chance to review the lawsuit. It said it is dedicated to the safety of its products and customers and is cooperating with NHTSA in its investigation.

Messages were left seeking comment from ARC and Ford. Volkswagen declined comment.

The plaintiffs allege that ARC's inflators use ammonium nitrate as a secondary propellant to inflate the air bags. The propellant is pressed into tablets that can expand and develop microscopic holes if exposed to moisture. Degraded tablets have a larger surface area, causing them to burn too fast and ignite too big of an explosion, according to the lawsuit.

The explosion can blow apart a metal canister housing the chemical, sending metal shards into the cabin. Ammonium nitrate, used in fertilizer and as a cheap explosive, is so dangerous that it can burn too fast even without moisture present, the lawsuit says.

The plaintiffs allege that ARC inflators have blown apart seven times on U.S. roads and two other times in testing by ARC. There have so far been five limited recalls of the inflators that totaled about 5,000 vehicles, including three recalls by GM.

Auto safety advocates say the case seems to mirror the Takata air bag saga that began in the early 2000s, which also involved exploding air bag inflators and resulted in 28 deaths worldwide, hundreds of injuries and the largest automotive recall in U.S. history. So far NHTSA has gathered information but hasn't forced any wider recalls from its investigation that began in July of 2015.

Sean Kane, president of Safety Research & Strategies Inc., which conducts research for lawyers that sue automakers as well as for other groups, noted that just as in the early stages of the Takata ordeal, many ARC ammonium nitrate inflators remain in use.

"It's almost like Groundhog Day here," said Kane, who asserts that NHTSA should have acted already. "It's not a question of whether it can kill or injure people. It already has."

ARC, the lawsuit alleges, knew about the danger of ammonium nitrate in patent applications it filed in 1995 and 1998. In 2019, after several ARC inflators blew apart, ARC acknowledged that its use was not acceptable for automotive air bags, according to the lawsuit.

The suit asserts that General Motors, which began recalling Takata ammonium nitrate inflators in 2013, should have known that ARC's inflators were also unstable.

"GM recalled only a small number of vehicles that contained a particular lot of inflators, despite its knowledge that ARC driver- and passenger-side inflators in various models and model years from 2002 through at least 2015 also had experienced ruptures," the lawsuit said.

In its statement Tuesday, GM said it makes recall decisions based on data and facts. It declined to comment further.

The lawsuit alleges that ARC's inflators are marred by a systemic problem rather than just a limited manufacturing defect. In 2014, an ARC inflator in a 2004 Kia Optima blew apart in a crash in New Mexico, injuring the driver.

Two years later, the driver of a car made by Kia's sister automaker Hyundai was killed in Canada when an ARC inflator exploded in a crash.

The lawsuit also names Volkswagen and Ford as defendants, alleging that they represented the air bag inflators as safe while knowing they were dangerous.

The National Highway Traffic Safety Administration began investigating ARC inflators in 2015 after an Ohio woman was injured when an inflator exploded in a Chrysler minivan. At the time the agency estimated that there were 490,000 ARC inflators on the nation's roads.

The review was elevated to an engineering analysis — a step closer to a product recall — in 2016 after the death in Canada.

Though a seven-year investigation is longer than most NHTSA reviews, inflators are particularly complex, said David Friedman, a former NHTSA acting administrator who is now a vice president at Consumer Reports.

Automakers appear to be balking at recalls for cost reasons, Friedman said. And NHTSA, he suggested, needs a "slam dunk" case before seeking recalls because of threats and lawsuits that automakers have filed in the past.

"That's one of the things that's broken in the system," he said.

Friedman described NHTSA as a chronically underfunded agency that has had to prioritize safety issues after four years of the Trump administration, which demanded far less federal regulation.

"The fact that it's stretched on seven years, the companies should have blinked, or NHTSA should have made them, or if they truly don't have a case, then say so," Friedman said.

A NHTSA official who spoke on condition of anonymity in order to speak freely about an ongoing inquiry, said the investigation has persisted because of the complexity of air bag inflators and because ARC's design is different from Takata's.

"We want to make sure what we do is thorough." he said.

He noted that NHTSA's investigation of ARC has had to examine issues different from the Takata case. Ammonium nitrate in Takata's inflators, for example, would deteriorate when airborne moisture entered the canister. But ARC pressurizes its inflator canisters to keep moisture out.

"It's not similar to Takata," the official said.

Whether the ammonium nitrate tablets can deteriorate without moisture is still being investigated, he said.

The agency, he said, has retrieved ARC inflators from vehicles to learn how they work. It also has gathered production and other data from ARC and automakers and issued an order making automakers report any problems with ARC inflators.

He noted that several years went by without any incidents, before there were three in the past two years and that each of those cases is informing the investigation.