Some commercial trucks may be among the vehicles in a massive recall of dangerously defective airbags, expected to be the largest recall in U.S. history.

After pressure from the U.S. Department of Transportation, air bag manufacturer Takata acknowledged that a defect exists in its air bag inflators and has agreed to a national recall of certain types of driver and passenger side air bag inflators.

These inflators were made with a propellant that can degrade over time. The degraded propellant ignites too quickly, producing excess pressure that causes the inflator to rupture and sends metal shards into the passenger cabin. The airbags have been blamed for six deaths worldwide.

The action essentially doubles the number of vehicles to be recalled to nearly 34 million. Ten major automakers (BMW, Fiat, Chrysler, Ford, General Motors, Honda, Mazda, Mitsubishi, Nissan, Subaru, and Toyota) had already issued recalls for approximately 17 million vehicles, and an 11th vehicle maker has been added to the list -– Daimler Trucks.

In a statement, Daimler Trucks North America said it "is presently investigating a very small population of potentially affected vehicles referenced in the Takata notices. The safety of our products is our highest priority and we are following this matter closely," said spokesman Dave Giroux.

Testing and investigation by Takata, auto manufacturers, and independent researchers have not yet established a definitive root cause of the inflator malfunctions. NHTSA’s analysis points to moisture infiltrating the defective inflators over extended periods of time as a factor.

Over time, that moisture causes changes in the structure of the chemical propellant that ignites when an air bag deploys.

Takata had previously argued that the problems were limited to regions with high humidity and had fought a national recall. This week's action expands the regional recalls that were limited to areas of high humidity to nationwide recalls.

Transportation Secretary Anthony Foxx called the recall "a monumental effort" and said it is "probably the most complex consumer safety recall in U.S. history."

The DOT's National Highway Traffic Safety Administration also has issued a Consent Order to Takata, requiring the company to cooperate in all future regulatory actions that NHTSA undertakes in its ongoing investigation and oversight of Takata. In addition, NHTSA announced its intent to begin a formal legal process to organize and prioritize the replacement of defective Takata inflators under the agency’s legal authority.

The DOT has established a new website, www.SaferCar.gov/RecallsSpotlight, to provide regular updates on the status of this and other recalls and of NHTSA’s investigation.