The probe – outlined in documents posted Monday by the National Highway Traffic Safety Administration – comes as a result of a petition by the Center for Auto Safety and follows previously announced safety reviews by the agency into the two carmakers over the potential fire risk posed by their vehicles.
The NHTSA said it has received over 3,100 complaints of fires between Hyundai and Kia, and over 100 reports of injuries.
In a statement, Hyundai said it has "engaged in frequent, open and transparent dialogue regarding non-collision engine fires and have provided the agency with comprehensive data on the issue."
Kia said it "openly shares information and data with NHTSA on all matters pertaining to vehicle safety, including this preliminary evaluation."
Combined, the two South Korean companies have recalled roughly 2.3 million vehicles over related issues.
For Kia, the investigation covers Optima, Optima Hybrid and Sorento models produced from 2011 to 2014, the Soul model made from 2010 to 2015 and the all-electric version of the crossover sports utility vehicle manufactured from 2014 to 2015.
The probe will cover Hyundai’s Sante Fe, Sonata and Sonata hybrid manufactured from 2011 to 2014, as well as the Sante Fe Sports from 2013 to 2014.