Stewart was unhurt, and plans to race Sunday in the NASCAR event at Watkins Glen, his manager Greg Zipadelli said, calling it "business as usual." The race is critical for his championship chances.
NASCAR says that nothing it has learned would preclude Stewart from racing Sunday.
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.
"He was visibly shaken by this accident," Povero said. "... This is right now being investigated as an on-track crash and I don't want to infer that there are criminal charges pending. 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 video of the crash at Canandaigua Motorsports Park showed driver Ward, wearing a black helmet and firesuit on a dimly lit track, walking toward Stewart's car before being hit and hurtled 50 feet. Povero said the the 25-lap race was under caution when Ward was struck. Stewart's car was behind another before he hit Ward.
"The first car swerved to avoid the driver," Povero said.
Ward, 20, was pronounced dead Saturday night at a hospital in Canandaigua.
Stewart-Haas Racing, of which Stewart is a co-owner, released a statement early Sunday morning.
"A tragic accident took place last night during a sprint car race in which Tony Stewart was participating," the statement read. "Tony was unhurt, but a fellow competitor lost his life. Our thoughts and prayers go out to his family and friends. We're still attempting to sort through all the details and we appreciate your understanding during this difficult time."
Povero said the 43-year-old Stewart, a three-time NASCAR champion and frequent competitor at local sprint car events, was questioned and released. The sheriff is asking for people who have video of the crash to contact the office so copies can be obtained for review.
"People that witnessed it were horrified," Povero said of the crash. "They were extremely shocked."
Video of the crash showed Ward, in the No. 13 car, spin into a wall after contact with Stewart's car. The video showed Ward climb quickly from his car and briskly walk around it in what appeared to be an attempt to confront Stewart as he passed by in his own car, Stewart's familiar No. 14.
The video showed Ward to the right of Stewart's car, which seemed to kick out from the rear and hit Ward. The driver was hurtled through the air and emergency personnel quickly reached Ward as he lay on the track.
Michael Messerly, a fan who witnessed the crash, said it appeared Stewart struck the driver as he tried to speed past him.
"The next thing I could see, I didn't see [the other driver] anymore," Messerly said. "It just seemed like he was suddenly gone."
The accident came just four days after the one-year anniversary of an accident in a sprint car race in Iowa in which Stewart 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 Saturday's 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.
NASCAR driver Dale Earnhardt Jr., whose father died as a result of an on-track accident, tweeted about the tragedy Sunday morning.
We will all loose someone in our time. When a loss is sudden and unexpected, the pain & sadness is suffocating. Prayers for the Ward family.
- Dale Earnhardt Jr. (@DaleJr) August 10, 2014Ward's website said he began racing go-karts in 1998 at age 4, but didn't start driving sprint cars until 2010. He was Empire Super Sprint rookie of the year in 2012 and this year was his fifth season racing in the Empire Super Sprints.
Stewart was involved in a July 2013 accident at Canandaigua Motorsports Park 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.
On Saturday, 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, almost a year after breaking his leg in the crash in Iowa. 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. 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.
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.
The four-team Stewart-Haas Racing Sprint Cup organization 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 is 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.
The Associated Press contributed to this report.
NASCAR Spokesman Says Stewart Can Race
NASCAR senior director of communications Kerry Tharp says there is nothing that precludes Tony Stewart from racing at Watkins Glen today.
Stewart Cooperating With Investigators
Ontario County (New York) Sheriff Philip C. Povero says Tony Stewart is cooperating with investigators and no criminal charges are pending after Stewart struck and killed a sprint car driver who had climbed from his car and was trying to confront him.
Tony Stewart To Race At Watkins Glen
Marty Smith says Tony Stewart plans to race at Watkins Glen after he struck and killed a sprint car driver who had climbed from his car and was on the track trying to confront him.
Craven Reacts To Stewart Incident
Ricky Craven reacts to the incident involving Tony Stewart, who struck and killed Kevin Ward Jr. after he climbed from his car and was trying to confront Stewart on Saturday night.