The Ontario County (N.Y.) Sheriff's Department was investigating an on-track altercation that involved Stewart and Kevin Ward Jr. According to The Charlotte Observer's Jim Utter, Ward was pronounced dead on arrival at a local hospital. Stewart is cooperating with authorities in the investigation, Utter tweeted.
Stewart's car allegedly struck driver Kevin Ward Jr., who had exited his sprint car after a crash involving Stewart, who frequently races on dirt tracks. The mishap occurred at about 10 p.m. at the half-mile track 30 miles southeast of Rochester.
Tyler Graves, a fellow racer and friend of Ward, told The Sporting News that he watching the action from the grandstands.
"Tony pinched him into the frontstretch wall, a racing thing," said Graves, 16, of Bolivar, N.Y. "The right rear tire went down, he spun on the exit of (Turn) 2. They threw the caution and everything was toned down. Kevin got out of his car. ... He was throwing his arms up all over the place at Tony for most of the corner.
"I know Tony could see him. I know how you can see out of these cars. When Tony got close to him, he hit the throttle. When you hit a throttle on a sprint car, the car sets sideways. It set sideways, the right rear tire hit Kevin, Kevin was sucked underneath and was stuck under it for a second or two and then it threw him about 50 yards."
Graves said Ward was not moving after he hit the ground.
Another witness, Adam Dulski, told USA Today, "It happened in Turn 2. The prior lap, Tony had gotten into him -- just spun him, nothing big, just spun him around. The caution came out. He hopped out of the car -- the driver of the 13. He hopped out to go and yell and point a finger at Tony, typical thing.
"Tony came around, the back end slid out and he definitely caught him -- I couldn't tell if it was with the front or the back of the car. The body made contact with the car and went sliding across the track. It was the worst thing I've ever seen."
Barbara Manning, a nursing supervisor at Thompson Health in Canandaigua, told Fox Sports that a man was at the hospital with life-threatening injuries, but she did not have a condition report and could not confirm that it was Ward.
"You never mean to do something like that," Graves told TSN. "Kevin was pissed and he let Tony know. And Tony was trying to give the message back that he wasn't happy either. He went over the line with it."
Police offered no additional details and the track cancelled the rest of the program.
"Tonight's racing events were suspended due to an accident," the track announced on Facebook. "Please hold off on any speculation until we have further information to share."
It then added, "Canandaigua Motorspots Park will not have an official statement on the accident that happened in the ESS race until tomorrow. Please pray for the entire racing community of fans, drivers, and families. Please be respectful in any comments."
Last July, Stewart was involved in another wreck at the same track that resulted in two drivers suffering injuries.
Stewart is scheduled to drive in the NASCAR Sprint Cup race on Sunday on the road course at Watkins Glen, N.Y.
Sports Xchange contributed to this report.