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.

He wasn’t the hot-tempered loudmouth who had taken stock-car racing by storm with three wins as a brash 28-year-old in 1999, following that with six victories a season later. No, as he rounded his 30s and entered his 40s, the stories about Stewart went on and on: He was introspective, wiser and calmer than he had once been — even if traces of his old habits lingered.

“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., not far from the Finger Lakes, Stewart participated in a sprint car event. Indulging his longtime hobby of racing on the rough-and-tumble smaller circuits the day before Sprint Cup races, 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 C. Povero told reporters early Sunday morning. After one car swerved to avoid Ward, Stewart’s car hit Ward, causing fatal injuries.

“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,” Ward’s family said in a statement released Sunday.

This July 5, 2014, photo provided by Empire Super Sprints, Inc., shows sprint car driver Kevin Ward Jr. in the vicotry lane with his car at the Fulton Speedway in Fulton, N.Y. Ward was killed Saturday at the Canandaigua Motorsports Park when the car being driven by Tony Stewart struck the 20-year-old who had climbed from his crashed car and was on the darkened dirt track trying to confront Stewart following a bump with Stewart one lap earlier. (Empire Super Sprints, Inc./Via AP)

In a second news conference Sunday afternoon, Povero 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 early 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, the races ending early, as emergency workers rushed toward Ward and authorities questioned Stewart. Povero told reporters Ward had been pronounced dead upon arrival at a nearby hospital and that Stewart was “fully cooperative.”

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

But also in the early-morning hours of Sunday, a video, apparently showing the incident, 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 then panning toward Stewart’s car stopped on the track in the distance.

Ward is not the first driver to put himself in harm’s way in a sport that has seen everything from helmet-throwing to fist fights. But the Canandaigua 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 in attendance to send photos and videos of the accident to help piece together what happened.

“The timing was unsafe,” driver Cory Sparks, a friend of Ward’s who was a few cars behind Stewart when Ward was killed, told the Associated Press. “When your adrenaline is going and you’re taken out of a race, your emotions flare.”

Tony Stewart did not race in Sunday’s NASCAR Sprint Cup event at Watkins Glen, N.Y., after the Saturday incident. (Derik Hamilton/AP)

Throughout his career, Stewart has been no stranger to on-track altercations that occasionally spilled onto pit road or into interviews. He punched a photographer in 2002, drawing a $50,000 fine. Stewart called out drivers, reporters and even fans, and sometimes his temper boiled over — Stewart occasionally has had to be restrained by members of his crew. “I’ll tell you, I don’t know what’s wrong with that boy,” analyst and former driver Darrell Waltrip said, leading Stewart to insult Waltrip during a post-race interview.

Stewart reportedly saw a psychologist for anger management and moved home to small-town Indiana, prompting some of those stories about his maturity. But deeper than the convenient story line, the hints were there that, as he was quoted as telling ABC News in 2006, he remained a “15-year-old trapped in a 34-year-old body.” As the years passed, he found himself in more anger-fueled confrontations. Stewart hurled his racing helmet at Matt Kenseth’s car during a race in 2012, and last year after a Sprint Cup event he followed Joey Logano to pit road, threw punches and had to be restrained. “I don’t enjoy getting mad like that,” he told reporters after the dust-up with Logano.

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 the television crews and media throngs. Racing is just racing there, without NASCAR’s strict rules and all of its off-track obligations. Not that he didn’t occasionally find trouble there, too. In July 2013, at the same track where Ward and Stewart crossed paths Saturday, 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 — Canandaigua Park on Saturday, Watkins Glen on Sunday.

“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.”