The US National Highway Transport Safety Agency (NHTSA) has opened formal investigations into almost three million vehicles sold by Hyundai and Kia in the USA.
Also being investigated is the 2010 to 2015 model year Kia Soul.
According to the NHTSA there have been 3125 reports of non-crash-related fires involving these models. One death and 102 injuries have been attributed to these vehicle fires.
The agency says the cars have been recalled in the past for engine fire-related issues, but its investigation will not be limited to engine components.
The investigations come after the Centre for Auto Safety, a consumer advocacy group, lodged a petition last year with the NHTSA and Congress.
A recent report by the Highway Loss Data Institute found the Sonata, Santa Fe, Optima and Sorento all had "higher non-crash fire claim rates" than similar vehicles from competitors.
It also found Hyundai and Kia models fitted with the company's 2.4-litre four-cylinder engine and 2.0-litre turbo had "elevated non-crash fire claim frequency" with fire risk going up "up dramatically as the vehicles aged".
The institute did not study figures for the Kia Soul as it was not originally included in the Centre for Auto Safety's petition.
Hyundai and Kia have told Reuters they are assisting with the investigation in an open manner.