Safety regulators in the United States have opened an investigation into the recall of almost 1.7 million vehicles issued by Hyundai and Kia in the country.

Reuters reports that the investigations come shortly after a South Korean whistleblower reported his concerns to the National Highway Traffic Safety Administration. That same whistleblower, Kim Gwang-ho, has already forced the government of South Korea to recall approximately 240,000 Hyundai and Kia vehicles.

U.S. regulators will investigate the timeliness of the three U.S. recalls and whether or not enough vehicles were covered.

Hyundai initially recalled 470,000 Sonata models in the country in 2015 due to a fault that could cause the engine to stall. In late March, an additional 572,000 Sonata and Santa Fe Sport models were caught up in the same issue and shortly after, Kia recalled 618,160 Optima, Sorento and Sportage vehicles.

Gwang-ho, a former employee, alleges that the engine at the center of the recalls wasn’t simply affected by poor manufacturing processes but is actually faulty.

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