GASTONIA, N.C. (AP) — The North Carolina house fire that killed NASCAR driver Denny Hamlin’s father and injured his mother has been ruled accidental but it’s still unclear what started it.

A report released by Gaston County Emergency Management & Fire Services said the Dec. 28 fire at the two-story home originated in a bedroom, but the “cause of ignition” is “undetermined.”

Otherwise, the investigation determined the fire as accidental, Gaston County spokesperson Adam Gaub wrote in an email Friday.

Hamlin’s parents, Dennis and Mary Lou Hamlin were found outside the home the evening of the fire suffering from catastrophic injuries, officials said. Dennis Hamlin, 75, later died, while his wife survived.

The home, located near Stanley about 20 miles (32 kilometers) northwest of Charlotte, was owned by a company that listed Denny Hamlin as its manager, according to government records.

Dennis Hamlin was already seriously ill last year, his son previously said.

Denny Hamlin is a leading driver in NASCAR’s top circuit, having won 60 NASCAR Cup Series races, including the Daytona 500 three times. Hamlin and Michael Jordan co-own the car driven by Tyler Reddick that won this year’s Daytona 500 last weekend.