The National Highway Traffic Safety Administration (NHTSA) said on Monday it would open a new safety defect investigation into three million Hyundai Motor Co. and Kia Motors Corp. vehicles for fires not caused by crashes.

The investigation is in response to a petition seeking a probe by the Center for Auto Safety. The auto safety agency since 2007 has been investigating some Hyundai and Kia vehicles for fire risks. The South Korean automakers have recalled more than 2.3 million vehicles since 2015 to address various engine fire risks.

Hyundai did not immediately comment, while Kia did not immediately respond to a request for comment.

NHTSA said the decision to initiate the additional safety defect probe was based on its analysis of information received from multiple manufacturers, consumer complaints and other sources.

Last month, Connecticut Attorney General William Tong said a group of U.S. states is investigating Hyundai and Kia for potential unfair and deceptive acts related to reports of hundreds of vehicle fires.

In November, Reuters reported that federal prosecutors had launched a criminal investigation into Hyundai and Kia to determine if vehicle recalls linked to engine defects had been conducted properly.

Reuters reported in January that the companies would offer software upgrades for 3.7 million vehicles not being recalled.

A South Korean whistleblower in 2016 reported concerns to NHTSA, which has been probing the timeliness of three U.S. recalls and whether they covered enough vehicles.

In 2015, Hyundai recalled 470,000 U.S. Sonata sedans, saying engine failure would result in a vehicle stall, increasing the risk of a crash. At the time, Kia did not recall its vehicles, which share the same “Theta II” engines.

In March 2017, Hyundai expanded its original U.S. recall to 572,000 Sonata and Santa Fe Sport vehicles with those Theta II engines, citing the same issue involving manufacturing debris. On the same day, Kia also recalled 618,000 Optima, Sorento and Sportage vehicles, all of which use the same engine.