The U.S. government’s road safety agency has paid more than $24 million to a whistleblower who reported that Hyundai and Kia moved too slowly to recall over 1 million vehicles with engine problems.

Concerns are mounting over Kia’s earlier recall of more than 137,000 Seltos and Soul models, as the NHTSA’s Office of Defects Investigation (ODI) now questions whether the fix is actually working.