Hyundai and Kia are under federal investigation over the massive recalls they issued in 2015 and 2017, with the prosecutors trying to determine whether or not the two Korean brands dealt with the issue in the best possible way.

The investigation could lead to hefty fines against Hyundai and Kia, which recalled nearly 1.7 million vehicles in the United States over an engine failure that increased the risk of an accident. This was one the biggest recalls in the history of Hyundai and its sister company.

Reuters cites a person with direct knowledge of the matter, saying that the U.S. Attorney Office for the Southern District of New York (SDNY) has launched a criminal investigation, in collaboration with NHTSA.

The first recall took place in 2015, when Hyundai brought in 470,000 vehicles fitted with the Theta II engine to fix them, as debris from the manufacturing process could cause various engine issues. Soon afterwards, an inside source from South Korea went to Washington and claimed that the issue was affecting much more cars. Eventually Hyundai recalled more cars for the same issue and Kia launched its own recall.

The affected models were Hyundai’s Sonata and Santa Fe, and Kia’s Optima, Sorento and Sportage.

Apart from the SDNY investigation, Hyundai and Kia are also under investigation by NHTSA over whether their recalls covered enough vehicles and if these were conducted in a timely manner.