The National Highway Traffic Safety Administration (NHTSA) is investigating a series of recalls covering 6.4 million Hyundai and Kia vehicles related to brake fluid leaks that may lead to fires, the safety regulator announced Monday.

The audit query will cover 16 individual recalls since 2016 for antilock braking system and Hydraulic Electronic Control Unit (HECU) issues manufactured by the same parts supplier and fires.

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NHTSA will evaluate the timeliness of the Korean automakers’ defect decision-making "and adherence with reporting requirements; and understand the varying defect descriptions and remedies between these recalls."

The safety regulator can issue an audit query if the manufacturer’s recall has a low completion rate.

In September, Hyundai and Kia recalled a combined 3.37 million vehicles in the U.S. after the auto manufacturers said internal brake fluid leaks can cause electrical shorts in affected models and could lead to fires.

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Hyundai’s recall covered 1.64 million Elantra, Genesis Coupe, Sonata Hybrid, Accent, Azera, Veloster, Santa Fe, Equus, Veracruz, Tucson, Tucson Fuel Cell and Santa Fe Sport vehicles from model years 2011 through 2015.

As of September, Hyundai said there have been 42 recall-related incidents since 2017, although no crashes, injuries or fatalities have been reported.

Meanwhile, Kia’s recall included 1.73 million Borrego, Cadenza, Forte, Sportage, K900, Optima, Soul Rio, Sorento and Rondo vehicles and covers model years from 2010 through 2017.

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Kia’s safety office confirmed one engine compartment fire in September, three localized fires and six localized melting incidents. There have been no crashes, injuries or fatalities related to the issue.

Dealers will replace the vehicles’ ABS and HECU fuses.

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Reuters contributed to this report.