UPDATED | November 4, 2018, 4:12 PM

FORT WORTH (CBSDFW.COM) – Two men were sent to the hospital in critical condition Sunday morning due to carbon monoxide poisoning at Texas Motor Speedway.

MedStar confirmed the two men were taken to hospitals in critical condition. One was taken to Texas Health Alliance in Fort Worth and the other was flown to Plano for hyperbaric treatment.

According to the Associated Press, Fort Worth Fire spokesman Kyle Clay said firefighters found the men shortly before 9 a.m. inside their converted bus, which was parked in the raceway’s infield.

Clay said a gas-powered generator inside the bus wasn’t properly ventilated and that both men appeared to be in their 50s.

The men’s identities have not been released as the investigation continues.

The MENCS AAA Texas 500 race is scheduled for 2 p.m. Sunday at TMS.