The United States National Highway Traffic Safety Administration (NHTSA) has now responded to the request sent by the Center for Auto Safety to initiate a probe into both Hyundai Motor Co and Kia Motors Corp vehicles for allegedly causing non-crash related fires. The safety regulator revealed this week that its investigation will include more than 3 million vehicles from both manufacturers.

The petition for an immediate investigation was submitted last year following more than 3,000 fires that have resulted in the injury of at least 100 people. The vehicles in question reportedly caught fire without warning while the engine was running. The NHTSA revealed that the investigation will cover Kia Optima sedans from 2011 to 2014, Kia Sorento SUVs from 2010 to 2015, Kia Soul compact SUVs from 2010 to 2015, Hyundai Sonata sedans from 2011 to 2014, and Sante Fe SUVs from 2011 to 2014.

The investigation follows a recent engine related recall by both companies, which the regulators believe may be related to the fires. The probe is meant to further shed light on the vehicles' safety concerns following the recalls. Hyundai and Kia previously issued a voluntary recall in 2015 for more than 2.3 million of their vehicles for various engine defects. The NHTSA revealed that the new investigation will be more extensive and will go beyond just checking engine components related to the previous recall.

According to the Center for Auto Safety, it sought the help of the federal agency following a number of consumer and manufacturer complaints pertaining to non-crash fires. The agency's executive director, Jason Levine, mentioned in an interview that it was long past time for the federal government to investigate both companies given the thousands of incidents that have already occurred. The agency hopes that the findings would lead to more recalls and hopefully a drastic improvement in the company's quality controls.

In response to the announcement, Hyundai Motor Group, which is also the parent company of Kia Motors, released a statement mentioning that it will be fully cooperating with the investigation. The South Korean automotive manufacturer also stressed that it is open to sharing any information and data to the NHTSA to assist in their investigation in all matters of vehicle safety.

In South Korea, prosecutors have launched their own investigation into the recalls, which included the summoning of company executives for questioning. Share prices for both companies fell slightly following the reports of the new NHTSA investigation. Hyundai Motor shares fell by around 2.5 percent, while Kia Motors shares dropped by 1.1 percent following the reports.