"It is expected that the scope of the current Takata recalls may expand as time goes on and will likely grow to include vehicles that are outside the scope of the current recalls," the letters said. "The recalls may also grow to include inflator types that are not currently within the scope of the Takata DIRs (defect information reports)."
Earlier this month, NHTSA said it now estimates 23.4 million defective airbag inflators are in U.S. vehicles.
The defective inflators can deploy with too much force in a crash, spewing metal fragments at vehicle passengers. The defective inflators are blamed for eight deaths in the United States.
The letters ask the automakers to spell out how many vehicles have Takata airbag inflators and whether they are considering recalls or any other service actions related to airbag inflators.
They also ask: "What challenges, if any, do you expect that you would face, or that the industry as a whole would face, if NHTSA issued an administrative order expanding the scope of the current Takata recalls, either by model year or inflator type?"
NHTSA will hold an October 22 public hearing regarding the Takata airbag recall.
The safety agency said it seeks to coordinate the different remedy programs by each automaker to ensure that defective vehicles and equipment are recalled and remedied quickly.
On September 16, NHTSA updated its list of affected manufacturers and vehicles.
Edmunds says: Consumers will have to wait until NHTSA is finished gathering information from these latest automakers before taking any action. In the meantime, it's a good idea to check NHTSA's VIN Lookup tool to see if your vehicle is included in the Takata airbag recall.