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

Hyundai, Kia Recall Background

In September of 2015, there was a recall. It was issuing by a Korean company. The recall took place after there were reports of engine failures and fires. Moreover, it has issued at least eight additional recalls. This has occurred for an assortment of engine issues. On Monday, the NHTSA documents were put on the agency’s website.

There were a total of 161 complaints receiving by the NHTSA. The complaints were about engine fires also in vehicles that were previously recalling. In fact, the agency has deepened its investigation into the Hyundai-Kia issues. Engineering analysis has been determining whether earlier recalls did include enough of the automaker’s vehicles.

In fact, the NHTSA’s probe is going to “monitor and evaluate the efficacy of the recall remedies for engine fire-related recalls. Also, the long-term viability of related programs and non-safety field,” this is according to a document released by the agency.

On Monday, Hyundai said that it is cooperating fully with U.S. regulators.

“Hyundai has taken on numerous proactive actions to address engine issues. They do include conducting many recalls. Plus launching a new engine monitoring technology. This would provide extended warranties and then enhance our customer service response,” the company said. “Hyundai does foster a culture of transparency and accountability as the safety of our customers which is a top priority in everything we do.”

On Monday, Kia did not immediately respond to a request for comment.