DETROIT (AP) — A Tennessee company is heading for a legal battle with U.S. auto safety regulators after refusing a request that millions of potentially dangerous air bag inflators be recalled.

The National Highway Traffic Safety Administration said Friday that ARC Automotive Inc. of Knoxville should recall 67 million inflators in the U.S. because they could explode and hurl shrapnel. At least two people have been killed in the U.S. and Canada, and six others have been hurt as a result of defective ARC inflators, the agency said.

The recall could cover nearly a quarter of the vehicles now on U.S. roadways.

In a letter posted Friday, the agency told ARC that it has tentatively concluded after an eight-year investigation that ARC front driver and passenger inflators have a safety defect.

But ARC responded that it no defect exists in the inflators, and that any problems were related to isolated manufacturing issues.

The next step in the process is for NHTSA to schedule a public hearing. It could then take the company to court to force a recall.

Also Friday, NHTSA posted documents showing that General Motors is recalling nearly 1 million vehicles equipped with ARC inflators. The recall covers certain 2014-2017 Buick Enclave, Chevrolet Traverse, and GMC Acadia SUVs.

The automaker says an inflator explosion “may result in sharp metal fragments striking the driver or other occupants, resulting in serious injury or death.”

Owners will be notified by letter starting June 25, but no fix is available yet. They’ll get another letter when one is ready.