The stories of Tony Stewart's maturation began nearly a decade ago: NASCAR's bad boy had finally mellowed, the quick anger and the brutal honesty mostly behind him.

"It was inevitable," the Associated Press quoted the three-time Sprint Cup champion as saying in 2011, "that I would eventually grow up."

On Saturday night, at a nondescript dirt track in Canandaigua, N.Y., Stewart, 43, tangled under the lights at Canandaigua Motorsports Park with a 20-year-old driver named Kevin Ward Jr. About halfway through the race, Stewart bumped Ward, who spun out. Ward left his car and walked onto the track, apparently to confront Stewart, Ontario County (N.Y.) Sheriff Philip Povero told reporters early Sunday morning. After one car swerved to avoid Ward, Stewart's car hit Ward, causing fatal injuries.

Povero later said no criminal charges had been filed, but investigators had interviewed Stewart in Canandaigua and were also sent to Watkins Glen, N.Y., where Stewart headed after the accident, intending to race in Sunday's Cheez-It 355. His Stewart-Haas team manager, Greg Zipadelli, called it "business as usual," but after an outcry, Stewart withdrew shortly before the race.

"There aren't words to describe the sadness I feel about the accident that took the life of Kevin Ward Jr.," Stewart said in a statement released Sunday afternoon. "It's a very emotional time for all involved, and it is the reason I've decided not to participate in today's race at Watkins Glen. My thoughts and prayers are with his family, friends and everyone affected by this tragedy."

Shortly after the incident, Canandaigua was closed as emergency workers rushed toward Ward. Povero told reporters Ward had been pronounced dead upon arrival at a nearby hospital and that Stewart was "fully cooperative."

Kevin Ward Jr. was hit and killed at New York’s Canandaigua Park Saturday after an altercation with Tony Stewart.

"At this very moment, there are no facts in hand that would substantiate or support a criminal charge or criminal intent," Povero said.

In the early-morning hours of Sunday, a video was posted on YouTube. It showed Ward's black No. 13 car sliding around one of the track's elbows, Stewart's No. 14 at Ward's left. The two cars seemed to touch, and Ward's car spun out and hit the wall; a moment later Ward left the car and stood on the half-mile track pointing and gesturing, apparently toward Stewart. When the No. 14 car made its way back around the track, Ward still standing there, Stewart's car made contact with Ward, whose body was blown backward as Stewart's car fishtailed. Ward was then seen lying motionless as emergency workers rushed toward him, the camera panning toward Stewart's stopped car in the distance.

The track was dimly lit and Ward was wearing a black suit, which might have contributed to the accident. It's one reason Povero is asking fans to send photos and videos to help piece together what happened.

"The timing was unsafe," driver Cory Sparks, a friend of Ward's, told the Associated Press. "When your adrenaline is going and you're taken out of a race, your emotions flare."

Stewart reportedly has seen a psychologist for anger management and moved home to small-town Indiana, prompting some of those stories about his maturity. But he hurled his racing helmet at Matt Kenseth's car during a race in 2012, and last year after a Sprint Cup event he threw punches at Joey Logano and had to be restrained. "I don't enjoy getting mad like that," he told reporters later.

Seemingly as an outlet to the high-pressure Sprint Cup circuit — where Stewart has earned nearly $116 million in winnings, not to mention endorsement dollars — he still found his way onto the dimly lit dirt tracks, away from NASCAR's strict rules. Yet in July 2013, also at Canandaigua, Stewart triggered a 15-car wreck that broke another driver's back. A month later, Stewart crashed at Southern Iowa Speedway, breaking two bones in his right leg and ending his racing season.

He returned to start the 2014 Sprint Cup season, signing up for dozens of sprint car races on top of his NASCAR schedule, including the double dip this past weekend.

"I learned that I was just getting too consumed by racing," Stewart told Forbes magazine in 2010, another of those stories about an older, mellower driver. "That's all I used to do was think about it. You have to be able to turn it off. You have to make yourself stop, or you'll get burned out."