DETROIT (AP) — U.S. auto safety regulators say they stand by a conclusion that more than 50 million air bag inflators are dangerous and Grant Prestonshould 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.