DETROIT — Hyundai and Kia have added more than a half-million vehicles to a 3½-year string of U.S. recalls for engine failures and fires.

Three recalls announced Thursday add new problems and vehicles to the Korean automakers’ list of safety woes, which have brought hundreds of complaints about fires from across the nation.

The companies have now recalled nearly 2.4 million vehicles for fire and engine failure problems since September of 2015, and they are under investigation by the National Highway Traffic Safety Administration for potentially being slow to fix faulty vehicles.

The largest of three recalls posted on the National Highway Traffic Safety Administration website Thursday covers nearly 379,000 Kia Soul small SUVs from 2012 through 2016 with 1.6-liter engines. Documents show that high exhaust gas temperatures can damage the catalytic converters, which control pollution. That can cause abnormal combustion and damage pistons and connecting rods. A failed connecting rod can pierce the engine block and cause oil leaks that can cause fires.

In addition, Hyundai and Kia are recalling 152,000 Tuscon SUVs from 2011 to 2013 and Sportage SUVs from 2011 and 2012 to fix an engine oil pan leak that also can cause fires.

All Souls with 1.6-liter engines made from July 8, 2011, through August 11, 2016, are being recalled. Letters will be mailed to owners starting April 12.

Hyundai owners will be notified starting March 29, while Kia owners will get letters starting April 10.

Jason Levine, executive director of the Center for Auto Safety, has said fire complaints to NHTSA have come from across the U.S., including a death in Ohio in April of 2017.