The National Highway Traffic Safety Administration (NHTSA) launched a probe Monday into the recalls of 6.4 million Hyundai and Kia vehicles due to brake fluid leaks that could cause fires.

The investigation will look into eight recalls made by each company over issues with their antilock braking systems (ABS) and Hydraulic Electronic Control Units (HECU), both of which were manufactured by the South Korean parts supplier Mando, Automotive News reported

The 16 individual recalls from Hyundai and Kia covered approximately 6.4 million vehicles, according to Fox Business. 

Hyundai Motor America and Kia America issued “park outside” recalls for more than 3.3 million vehicles in September due to the risk of fire, with the NHTSA saying at the time that “the safest place to park them is outside and away from homes and other structures.”

The fires were traced back to internal brake fluid leaks that could sometimes cause electrical shorts, leading to flames.

The NHTSA is set to audit the Korean automakers’ defect decision-making “and adherence with reporting requirements; and understand the varying defect descriptions and remedies between these recalls.”

Federal regulators have the ability to issue an audit query if the manufacturer’s recall “has a low completion rate,” Fox Business reports. 

The September recall covered Hyundai’s Elantra, Genesis Coupe, Sonata Hybrid, Accent, Azera, Veloster, Santa Fe, Equus, Veracruz, Tucson, Tucson Fuel Cell, and Santa Fe Sport models from 2011 through 2015.

At that time, Hyundai reported 42 recall-related incidents since 2017, though no crashes, injuries, or deaths occurred. 

Kia’s recall included Borrego, Cadenza, Forte, Sportage, K900, Optima, Soul Rio, Sorento, and Rondo models from 2010 through 2017.

Kia reported that one engine compartment fire occurred in September, as well as three localized fires and six localized melting incidents. There also were no crashes, injuries, or fatalities related to the recall reported.

Affected ABS and HECU fuses will be replaced by dealers.