The top U.S. auto-safety regulator told lawmakers Wednesday there is no way Takata Corp. can justify limiting an air-bag recall to only high-humidity states.

David Friedman, the deputy administrator for the National Highway Traffic Safety Administration, cited data that show humidity is less of a factor than first thought in the malfunction risk for driver's side air bags. He said NHTSA will hire within a week an independent expert to conduct more air bag tests.

"While it's true the automakers implement the actual recall, Takata is making the defective products and Takata should be declaring nationwide a defect in their products," Friedman told reporters Wednesday after testifying to Congress. "That will lead to a recall from all the automakers."