IndyCar officials say British driver Dan Wheldon was killled after colliding with a fence pole that lined the Las Vegas Motor Speedway track.
Wheldon died aged 33 following a multi-car pile-up in the final race of the 2011 season on 16 October.
InyCar say the pole "intruded in the cockpit and made contact with the driver's helmet and head."
Critics had previously questioned the race's safety considerations, including the number of cars allowed to compete.
But officials say the starting field of 34 cars was "deemed to be acceptable" and that the accident could have occurred with "any size starting field at any track".
Wheldon, who was bidding to win a $5m jackpot by coming from last on the starting grid to win in Las Vegas, was one of 15 drivers involved in a crash at the second corner on lap 13.
After contact between two cars further ahead in the field, Wheldon's crash was sparked by a spin from Vitor Meira that led to a collision between EJ Viso and Charlie Kimball.
IndyCar's accident report, which took two months to compile, found that Wheldon decelerated from 224 mph to 165 mph just seconds before running into Kimball.
Wheldon's number 77 car then "travelled in a nose-up, semi-airborne state for approximately 325 feet" before hitting the pole.
Wheldon then suffered "two distinct head forces", the second of which "produced the non-survivable blunt force injury traumas to Dan's head".