Hyundai and Kia vehicles are under investigation by U.S. auto safety regulators for reports of engine failures and fires that have followed the manufacturer for over six years. More than 3 million cars from model years 2011 through 2016 are subject to the probe by the National Highway Traffic Safety Administration (NHTSA).

The Korean company issued a recall in September of 2015 after reports of engine failure and fires. It has issued at least eight additional recalls since for various engine issues, according to NHTSA documents posted Monday on the agency's website.

The NHTSA has received a total of 161 complaints about engine fires, even in vehicles that were previously recalled. The agency deepened its investigation into the Hyundai-Kia issues by opening an engineering analysis to determine whether earlier recalls included enough of the automaker's vehicles.

The NHTSA's probe will also "monitor and evaluate the efficacy of the recall remedies for engine fire related recalls, as well as the long-term viability of related programs and non-safety field," according to a document released by the agency.

More recalls of Hyundai-Kia vehicles could result from the engineering analysis.

Hyundai Vehicle
Hyundai and Kia vehicles are under investigation by the National Highway Traffic Safety Administration for reports of engine failures and fires in more than 3 million cars from model years 2011 through 2016.Justin Sullivan/Getty Images

Messages were left early Monday seeking comment from Hyundai and Kia.

The vehicle fires involve the related Korean automakers' Theta II GDI, Theta II MPI, Theta II MPI hybrid, Nu GDI and Gamma GDI engines. Models covered include Hyundai's Sonata, Santa Fe and Elantra and as well as Kia's Sorento, Rio, Optima and Soul. Model years covered are 2011 through 2016.

The agency says three people have reported eye and burn injuries that did not require medical treatment.

In November of 2020, NHTSA announced that Kia and Hyundai must pay $137 million in fines and for safety improvements because they moved too slowly to recall more than 1 million vehicles with engines that can fail. The fines resolve a previous probe into the companies' behavior involving recalls of multiple models dating to the 2011 model year.

Kia was to pay $27 million and invest $16 million in safety performance measures. Another $27 million payment will be deferred as long as Kia meets safety conditions, NHTSA said.

Kia denied the U.S. allegations but said it wanted to avoid a protracted legal fight.

Data collected by the nonprofit Center for Auto Safety show 31 U.S. fire and engine-related recalls from Hyundai and Kia since 2015. The recalls involve more than 20 models from the 2006 through 2021 model years totaling over 8.4 million vehicles.

Many of the recalls involved manufacturing defects that stopped oil from flowing through the engine block. Many involved expensive engine replacements.

Hyundai and Kia also did a U.S. "product improvement campaign" covering 3.7 million vehicles to install software that will alert drivers of possible engine failures.

The Associated Press contributed to this report.