US NASCAR driver Tony Stewart struck and killed a sprint car driver who was walking on a dirt track during a race in upstate New York on Saturday night.
Ontario County Sheriff Philip Povero said his department's investigation is not criminal and that Stewart was "fully cooperative" and appeared "very upset" over what had happened.
Povero would not identify the driver Stewart hit, but said he was pronounced dead Saturday night at a hospital in Canandaigua. Stewart was unhurt.
A video of the crash at Canandaigua Motorsports Park showed driver Kevin Ward Jr, clad in a black helmet and firesuit on a dimly lit track, walking toward Stewart's car before being hit and hurtled 50 feet.
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.
"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."
Povero said the 43-year-old Stewart, a frequent competitor at local sprint car events, was questioned and released. The sheriff is asking for people who recorded video of the crash to provide copies for investigators to review.
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."
WARNING: GRAPHIC VIDEO
Stewart is scheduled to race in in NASCAR's event Sunday at Watkins Glen. The statement did not address whether Stewart would remain in the race, which is critical for his championship chances.
The accident came just four days after Stewart marked the one-year anniversary of an accident in a sprint car race in Iowa, where he suffered a compound fracture to his right leg. The injury cost him the second half of the NASCAR season.
The track, about 30 miles southeast of Rochester, canceled the remainder of the race within five minutes and later posted a message on its Facebook page encouraging fans to "pray for the entire racing community of fans, drivers, and families."
It said a statement on the crash would come later Sunday.
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.
Stewart was involved in a July 2013 accident at Canandaigua 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.
Messerly said Saturday's crash appeared to be the result of "a number of bad decisions" and not "any intent on Tony Stewart's part."
"Auto racing is a dangerous sport," he said. "I think it was just an unfortunate set of circumstances that created a tragic situation."
Ambulances, fire trucks and police arrived within minutes of the crash, Messerly said. Fans filed out in stunned silence, he said.
Stewart only returned to sprint track racing last month. He didn't return to racing in any form until February when preparation for NASCAR's season-opening Daytona 500 began.
He refused to stop his extracurricular racing despite the injury and was scheduled to race Sunday.
Stewart was a spectator at the Knoxville Nationals in Iowa on Tuesday, the one-year anniversary of the accident, and posted on his Twitter account: "Thank you to everyone that worked so hard to get me back to where I'm at today. It's your life, live it!"
Roughly three hours after the accident in New York, Donny Schatz, a sprint car driver for Tony Stewart Racing, won the prestigious Knoxville Nationals in Iowa for an eighth time. Stewart had spent much of the earlier part of the week trackside in Iowa watching his drivers compete.
"I was just told there was an incident involving Tony. I don't know to what extent or what's happening," Schatz said.
Stewart is co-owner of Stewart-Haas Racing, the four-team Sprint Cup organisation fields cars for Stewart, Kevin Harvick, Kurt Busch and Danica Patrick. He's struggled a bit this year since returning, and heads into Sunday's race winless on the season and ranked 19th in the standings.
Stewart was scheduled to start 13th on Sunday at Watkins Glen International in south central New York state. He has just five races remaining to either score a win or move inside the top 16 in points to grab a valuable spot in NASCAR's championship race.