(CNN) -- Although NASCAR driver Tony Stewart is not currently facing criminal charges in the death of another driver, he is not officially in the clear.
"This is an ongoing investigation, and all options remain available," Sheriff Philip C. Povero of Ontario County, New York, said Monday. "We're continuing to gather and analyze every piece of evidence we can."
Stewart hit and killed another driver who was walking on a track during a dirt-track race in upstate New York on Saturday night, authorities said.
Kevin Ward Jr., 20, died from injuries suffered in the incident at the Empire Super Sprints series event at the Canandaigua Motorsports Park.
The track is about a two-hour drive from Watkins Glen International race track, where Stewart had been scheduled to race in a NASCAR event Sunday.
"There aren't words to describe the sadness I feel about the accident that took the life of Kevin Ward Jr.," Stewart said Sunday, according to a written statement tweeted by NASCAR. "It's a very emotional time for all involved. ... My thoughts and prayers are with his family, friends and everyone affected by this tragedy."
Stewart decided not to take part in Sunday's event.
"Our hearts go out to Kevin's family," said Stewart's team director, Greg Zipadelli. "This is a very tough, very emotional time for everybody -- his family, our family at Stewart-Haas Racing."
On Saturday night, Ward's sprint car hit the outside wall during lap 14 of a 25-lap race, Povero said.
A video of the incident shows two cars coming out of a turn with Stewart's No. 14 car sliding up the track toward Ward's No. 13 car. The two cars get close and appear to make contact before Ward's car hits the wall and spins out. Another car narrowly avoids hitting the 13 car as its sits on the track facing the wrong direction.
Ward gets out of his crashed car, walking on the track toward the racecars, which had slowed for a yellow flag. Ward points a finger and appears to be yelling. One car swerves to avoid Ward on the half-mile dirt track.
Stewart's car passes close to Ward, and it appears that its right rear tire hits him.
An ambulance took Ward to a hospital, but he died before it arrived, Povero said.
"Stewart has fully cooperated" with an investigation, Povero said. "He acknowledges the collision with the driver." It appeared to be an on-track incident, Povero said.
Ward, from Port Leyden, New York, was in his fifth season in the Empire Super Sprints series. He began racing when he was 4, running go-kart events.
Stewart's spokesman said officials are trying to sort out the details.
"A tragic accident took place last night during a sprint car race in which Tony Stewart was participating," Mike Arning said. "Tony was unhurt, but a fellow competitor lost his life. Our thoughts and prayers go out to his family and friends."
A competitor that runs the car that swerved around Ward expressed sympathy.
"Our deepest condolences go to the entire Ward Family. We are one huge ESS family, you will be truly missed bud! #RIP13," Hebing Racing tweeted.
Stewart is a three-time champion in NASCAR's top division and won a sprint car championship in 1995.
"Our thoughts and prayers go out to the family, friends, and fellow competitors of Kevin Ward Jr.," NASCAR said in a written statement Sunday. "We support Tony Stewart's decision to miss today's race, and we will continue to respect the process and timeline of the local authorities and will continue to monitor this situation moving forward."
Stewart also owns a dirt race track in New Weston, Ohio, and has frequently raced sprint cars during the NASCAR season. He broke a leg last year in a crash that ended his racing season. He has raced the full NASCAR season this year but only returned to sprint cars in July.
CNN's Chris Welch, Jason Durand and Stephanie Gallman contributed to this report.