Kevin Ward Jr. had crashed following contact with Stewart one lap earlier and got out of his car as it was stopped along the fence. Video of the incident showed Ward walking from his crashed car onto the racing surface as cars circled by, and, as he gestured at Stewart's passing car, he was struck.
Authorities questioned Stewart but said no criminal charges were imminent.
Investigation UnderwayOntario County Sheriff Philip Povero said Stewart was "visibly shaken" and had been cooperative in the investigation. Authorities were asking spectators and others to turn over any video they recorded of the crash.
"This is right now being investigated as an on-track crash and I don't want to infer that there are criminal charges pending," Povero said. "When the investigation is completed, we will sit down with the district attorney and review it. But I want to make it very clear: there are no criminal charges pending at this time."
A witness said it appeared Ward was trying to confront Stewart, the three-time NASCAR Sprint Cup Champion. The video showed Ward standing to the right of Stewart's familiar No. 14 car, which seemed to kick out from the rear and hit him.
Moments earlier, Ward and Stewart were racing side-by-side for position as they exited a turn. Ward was on the outside when Stewart, on the bottom, seemed to slide toward Ward's car and crowd him toward the wall. The rear tire of Stewart's car appeared to clip the front tire of Ward's car, and Ward spun into the fence.
'He Was Suddenly Gone'Povero said Ward, who was wearing a black firesuit and black helmet, had walked into the racing area and one car swerved to avoid him before he was struck by Stewart.
"The next thing I could see, I didn't see [the other driver] anymore," witness Michael Messerly said. "It just seemed like he was suddenly gone."
A spokesman for Stewart's racing team called Ward's death a "tragic accident."
"Our thoughts and prayers go out to his family and friends," the spokesman said in a statement. "We're still attempting to sort through all the details."
The dirt track, about 30 miles southeast of Rochester, canceled the remainder of the race and later posted a message on its Facebook page encouraging fans to "pray for the entire racing community of fans, drivers, and families."
Ward's website said he began racing go-karts in 1998 at age 4, but didn't start driving sprint cars until 2010. The 20-year-old from Port Leyden, New York, was Empire Super Sprint rookie of the year in 2012 and this year was his fifth season racing the Empire Super Sprints.
Not the First CrashStewart often competes in extracurricular events. The multimillionaire is known to participate in races with purses worth less than $3,000 and drive alongside drivers of varying ages and talent levels.
The accident Saturday came almost exactly a year after Stewart suffered a compound fracture to his right leg in a sprint car race in Iowa. The injury cost him the second half of the NASCAR season. Stewart only returned to sprint track racing last month, and won in his return, at Tri-City Motor Speedway in Michigan.
The site of Saturday night's crash is the same track where Stewart was involved in a July 2013 accident that seriously injured a 19-year-old driver. He later took responsibility for his car making contact with another and triggering the 15-car accident that left Alysha Ruggles with a compression fracture in her back.
Stewart will not race Sunday. Greg Zipadelli, competition director for Stewart-Haas Racing, said the three-time NASCAR champion "feels strongly" he should not race at Watkins Glen following Kevin Ward Jr.'s fatal accident.