DETROIT (AP) — U.S. auto safety regulators say they stand by a conclusion that more than 50 million air bag inflators are dangerous and should not be in use, taking another step toward a massive recall.

The decision Wednesday by the National Highway Traffic Safety Administration involves inflators made by ARC Automotive Inc. in Tennessee and another parts manufacturer. It comes despite opposition from automakers.

The inflators in about 49 million vehicles from 13 manufacturers can explode and hurl shrapnel into drivers and passengers.

The agency has said the inflators are responsible for at least seven injuries and two deaths in the United States and Canada since 2009.

NHTSA said seven of the inflators have blown apart in the field in the U.S., each showing evidence of insufficient welds or too much pressure in a canister designed to contain the explosion and fill the air bags in a crash.

In addition, the agency said 23 of the inflators have ruptured in testing with causes common to the inflators that blew apart in the field. Also, four inflators have ruptured outside the U.S., killing at least one person, the agency said.

"To be sure, the overwhelming majority of the subject inflators will not rupture upon deployment," NHSTA wrote. "However, based on the evidence linking past ruptures to the same friction welding process, all of the subject inflators are at risk of rupturing."

Multiple automakers argued in public comments that NHTSA did not establish a safety defect and that none of the millions of inflators in their vehicles have ruptured.

But NHTSA said the only way to know which of the ARC-designed inflators will blow apart is for them to deploy in a crash. The federal motor vehicle safety act "does not allow such a defect to go unaddressed," the agency said.

The agency will take comments again for 30 days, then use them to make a final decision on whether to pursue the massive recall. The government could wind up suing ARC in an effort to force a recall.

A message was left Wednesday seeking comment from ARC of Knoxville.

Regulators asked ARC in May of 2023 to recall the inflators, but the company refused to issue a full-scale recall, setting the stage for the possible court fight.

NHTSA held a public hearing on the dispute last October, where it appeared ready to seek the recall due to a manufacturing flaw that could send metal shrapnel rocketing through a car's interior.

At the heart of the issue is the metal inflator canister inside the airbag device. The government contends that a crucial flaw could cause this canister to "rupture" upon impact. Instead of releasing pressurized gas to inflate the air bag, the canister essentially explodes, sending metal shrapnel into the vehicle at head height.

NHTSA contends that byproducts from welding during manufacturing can clog a vent in the canister that is designed to let gas escape to quickly fill air bags. In the defective products, pressure can build to the point where the canister is blown apart.

Injuries caused by the shrapnel can be gruesome, and the inflators pose an unreasonable risk of serious injury or death to drivers and passengers, the agency has said.

But ARC and many automakers oppose a recall, with several manufacturers saying the problem happens so infrequently that NHTSA has not established that a safety defect exists.

Fiat Chrysler Automobiles, a unit of Stellantis, for instance, wrote in comments that there has never been an inflator rupture in an FCA vehicle that matches NHTSA's theory on why the inflators are defective. The company said it has 4.9 million older vehicles on the road with driver and passenger inflators made by ARC.

An inflator in a 2002 Chrysler Town and Country minivan did blow apart in 2009, but ARC determined this one had a unique cause not found in other ruptures, FCA wrote. "In the 14 years following that event, there have been no further incidents in an FCA U.S. vehicle," the company wrote.

Vehicles from BMW, Stellantis, Ford, General Motors, Hyundai, Jaguar-Land Rover, Kia, Maserati, Mercedes, Porsche, Tesla, Toyota, and Volkswagen all have inflators made by ARC or parts supplier Delphi. A complete list of vehicles with the inflators has not been released. Delphi made about 11 million of the inflators under license from ARC.