Nascar star Tony Stewart struck and killed a sprint car driver who had climbed from his vehicle and was on the track during a race in New York state.
Ontario County Sheriff Philip Povero said his department's investigation is not criminal and that Stewart was "fully co-operative" and appeared "very upset" over what had happened.
"He was visibly shaken by this accident," Mr Povero said.
A video of the crash at Canandaigua Motorsports Park showed 20-year-old driver Kevin Ward wearing a black helmet and firesuit on a dimly lit track, walking towards Stewart's car before being hit and hurtled 50 feet.
Mr Povero said 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," Mr Povero said.
He said the driver was pronounced dead last night at a Canandaigua hospital. Stewart was unhurt.
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 number 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) any more," witness Michael Messerly said. "It just seemed like he was suddenly gone."
Mr Povero said 43-year-old Stewart, a frequent competitor at local sprint car events, was questioned and released. The sheriff asked 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."
Stewart's racing team manager Greg Zipadelli said Stewart will race in Nascar's event today at Watkins Glen, calling the race "business as usual".