Irvin Favre, 58, went off the road at 5:23 p.m. near Kiln, the hometown of Brett, a three-time NFL most valuable player. Irvin Favre ran into a ditch, said Sgt. Joe Gazzo of the Mississippi State Highway Patrol.
"It didn't appear that the accident was serious enough to cause him to be unconscious, so that leads us to believe that a medical condition was what caused him to go off the road," Gazzo said.
Irvin Favre was pronounced dead at 6:15 p.m. and an autopsy will be performed Monday.
"First, the witnesses, then the ambulance, and then at the hospital they tried to revive him," Gazzo said. "It's going to be a great loss to the community, he was a great guy."
Favre's status for Monday night's crucial game against the Oakland Raiders would be determined Monday, Green Bay spokesman Aaron Popkey said from San Francisco.
"Our thoughts are with Brett and his family," Popkey said.
Favre has started 204 consecutive games, including playoffs, an NFL record for quarterbacks. The Packers trail Minnesota by a half-game in the NFC North.