Tony Stewart, a three-time champion at NASCAR’s highest level, was moonlighting Saturday night. He had gone back to his racing roots for a few hours, competing in a local dirt-track event at Canandaigua Motorsports Park in upstate New York.

There was nothing unusual about Stewart’s presence there, or about the bump he delivered to Kevin Ward Jr.’s sprint car that sent it spinning into the outside wall. And it was not all that surprising to see Ward, 20, unbuckle his seat belts, jump out of the car and look to confront Stewart on the track. Stewart himself had done that to a driver in a 2012 NASCAR race.

But what happened next was hard for anyone to fathom, though available for the world to see in a widely circulated video. As Ward stood on the track and pointed toward Stewart, Stewart’s sprint car fishtailed, the right tire hitting Ward and dragging him under the car. Ward was thrown several feet and lay motionless on the track. He was later declared dead at a hospital.

The accident left a racing community in mourning, a NASCAR star’s future in question and a sport under the microscope for its history of verbal and often violent confrontations between adrenaline-fueled drivers.

Save 75% on a Digital Subscription Today

The death occurred about an hour away from Watkins Glen International, where Stewart was supposed to compete in a NASCAR Sprint Cup Series race Sunday. But after his team had said it would be “business as usual,” Stewart announced he would not compete.

“There aren’t words to describe the sadness I feel about the accident that took the life of Kevin Ward Jr.,” Stewart, 43, said in a statement.

The accident is being investigated by the Ontario County Sheriff’s Department, which said Stewart was cooperating. No criminal charges have been filed.

“Of particular interest at this time is forensic examination of any videos that exist of this crash that occurred last evening,” Sheriff Philip C. Povero said. “We’re also finishing a law-enforcement reconstruction of the crash.

“At this very moment, there are no facts at hand that would substantiate or support a criminal charge or indicate criminal intent on the part of any individual.”

Ward’s family issued a statement, saying, “We appreciate the prayers and support we are receiving from the community, but we need time to grieve and wrap our heads around all of this.”

Witnesses offered conflicting opinions about the accident, some blaming Ward for approaching Stewart, or the darkness of the track. Still others pointed to Stewart.

Although it seemed Ward put himself at grave risk by walking onto the track while cars were on it, that kind of action has precedent. Drivers have confronted other drivers directly on the track.

In 2012, Stewart walked onto the track in Bristol, Tenn., after a wreck with Matt Kenseth and threw his helmet at Kenseth’s car.