Investigators from the National Highway Traffic Safety Administration are delving into stalling problems in nearly 1.7 million Hyundai and Kia vehicles. They are following up on warnings from a whistleblower who had access to documents about the safety defect.

Earlier recalls over the stalling defect may not have included all the faulty cars. According to Reuters, “Kim Gwang-ho, then an engineer at Hyundai, flew to Washington in August 2016 to tell NHTSA the companies should have recalled more vehicles over the problem, citing an internal report. He also reported several alleged safety lapses to both U.S. and South Korean authorities.

On March 31, Hyundai expanded its original U.S. recall to 572,000 Sonata and Santa Fe Sport vehicles with “Theta II” engines, citing the same issue involving manufacturing debris, the NHTSA said. On the same day, Kia also recalled 618,160 Optima, Sorento and Sportage vehicles which use the same engine.”

Stalling in traffic is a serious safety defect, which can lead to a crash, injuring or killing the driver, passengers, and others who share the roads.

If your car intermittently stalls in traffic, or has another safety defect, here is where to file a complaint with the National Highway Traffic Safety Administration. NHTSA has the authority to pressure auto manufacturers to issue safety recalls and to provide repairs at no cost to consumers, for up to 15 years after the recall is issued.

File an auto safety complaint with the NHTSA

Read more: Reuters: U.S. Regulators Open Probe into nearly 1.7 million Hyundai, Kia models