The Takata Corp. has agreed to expand the national recall of its defective air bag inflators, bringing the number of vehicles covered to nearly 34 million, U.S. Transportation Secretary Anthony Foxx announced.

Takata’s action, following months of growing pressure from the National Highway Traffic Safety Administration, makes the vehicle safety recall the largest ever in the U.S. 

The recall expansion essentially doubles the number of vehicles covered. The goal is to replace certain types of driver- and passenger-side air bag inflators made with a “propellant that can degrade over time and has led to ruptures that have been blamed for six deaths worldwide,” the National Highway Traffic Safety Administration said in a released statement.

The agency is waiting for the automakers to supply a complete list of vehicles affected by the recall expansion. When the information becomes available, it will be released publicly.

“We are pleased to have reached this agreement with NHTSA, which presents a clear path forward to advancing safety and restoring the trust of automakers and the driving public,” Takata CEO Shigehisa Takada said in a statement.

NHTSA also issued a consent order requiring Takata to cooperate with the agency’s future regulatory actions tied to the recall investigation and oversight. The federal safety agency, part of the Department of Transportation, is taking steps to assume legal authority over management of the safety recall. NHTSA intends to organize and prioritize replacement of Takata’s defective air bag inflators.

The actions expand regional recalls of Takata passenger-side inflators, currently limited to areas of high absolute humidity, to nationwide recalls involving more than 16 million vehicles. Additionally, the current nationwide recall of driver-side inflators will expand to more than 17 million vehicles.

“It’s anticipated that the remedy of vehicles will be prioritized based upon risk, with the vehicles that present the greatest risk in terms of age and geographic location to be serviced first,” NHTSA said.

Despite investigations and testing by Takata, automakers and independent researchers, a definitive root cause for the inflator malfunctions still hasn’t been established, NHTSA said. But the problem appears to be related to moisture infiltrating the defective inflators over extended periods of time.

Eventually, that moisture causes changes in the structure of the chemical propellant that ignites when an air bag deploys. The degraded propellant ignites too quickly, producing excess pressure that causes the inflator to rupture. The resulting metal shards can strike vehicle occupants and cause serious injury or death.

NHTSA has already held informal discussions with automakers and parts suppliers in “an effort to coordinate one of the largest and most complex product recalls in history,” the agency said.

Ten automakers – Honda, BMW, Fiat Chrysler (FCA), Ford, General Motors, Mazda, Mitsubishi, Nissan, Subaru and Toyota – have previously issued recalls to replace the air bag inflators. The recall expansion is expected to add Daimler Trucks to the list of affected manufacturers.

“From the very beginning, our goal has been simple: a safe air bag in every vehicle,” said NHTSA Administrator Mark Rosekind. “The steps we’re taking today represent significant progress toward that goal.”