HARTFORD, Conn. — Hyundai Motor Co. and Kia Motors Corp. face new investigations into a spate of mechanical issues, including engine fires, that has led to the recall of millions of vehicles sold in the U.S. On Monday, Connecticut Attorney General William Tong announced the launch of an investigation by a cohort of unnamed states, each of which has fielded numerous complaints from Hyundai and Kia owners.

“My office is one of the leaders of an ongoing multistate investigation into deeply troubling reports of spontaneous fires in Hyundai and Kia vehicles,” Tong said in a statement. “We are aware of multiple fires involving Connecticut vehicles, including some allegedly already repaired through the recall process. This is a serious matter, and we are moving aggressively and responsibly to uncover the facts and to ensure accountability.”

The same day, U.S. Sen. Richard Blumenthal, also of Connecticut, was set to formally request a new investigation by the National Highway Safety Traffic Administration. In a widely publicized letter sent to Blumenthal last month, Hyundai Motor America officials acknowledged “sporadic and transient engine manufacturing issues” and outlined steps it has taken to resolve consumer concerns, including extending engine warranties and monitoring field performance of serviced and replaced engines.

More than 2.3 million units built by the South Korean manufacturers have been recalled at least once since 2015, most powered by some variation of the “Theta II” series of direct injection gasoline engines. Government officials and consumer advocacy groups, including the Center for Auto Safety, have pressured Hyundai and Kia to recall millions more.

In response to the reports, Hyundai released a statement late Monday assuring consumers the factory is “fully cooperating with the government in this matter, and is committed to providing American motorists with safe, high quality, efficient, and affordable vehicles.”

In January, former Kia Motors America warranty auditor and adjuster Jason Vaughn told Auto Dealer Today he uncovered the cause of the fires in August or September of 2017. He said he unsuccessfully lobbied management to investigate before ultimately resigning, registering as a whistleblower with the U.S. Securities and Exchange Commission, and going public.

“My opinion now is that, from the mid-management level and up, Kia knew about the fires and there was a lot of damage control going on,” Vaughn told ADT in January.

A KMA spokesman refuted Vaughn’s accusations in an email to the magazine. “Kia Motors America is a responsible company and takes all reported concerns seriously,” he wrote, in part.