A video of the crash at Canandaigua Motorsports Park showed driver Kevin Ward jnr walking toward Stewart's car before being hit and thrown 15 metres.

Povero said Stewart, 42, 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 reviewed.

People that witnessed it were horrified, Povero said of the crash. They were extremely shocked.

Calls to Stewart's representatives were not immediately returned, and he was scheduled to race in in NASCAR's event at Watkins Glen on Sunday.

There was no word on whether Stewart would pull out of the race, which is critical for his championship chances.

Michael Messerly, a fan who saw the crash, said it appeared Stewart hit a driver who was walking on the dimly lit track after they had collided on the previous lap.

He said Stewart struck the driver as he tried to speed past him.

I didn't see [the other driver] any more, he said. It just seemed like he was suddenly gone.

The accident came just four days after Stewart marked the one-year anniversary of an accident in a sprint car race in Iowa, in which he suffered a compound fracture to his right leg. The injury cost him the second half of the NASCAR season.

The track, about 48 kilometres south-east of Rochester, cancelled the rest 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 on Sunday.

Ward'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 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 at an Iowa track. 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, a three-time NASCAR champion, 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.

AP