Dan Wheldon has been airlifted to University Medical Centre in Las Vegas following a huge 15-car accident in the early stages of the IndyCar Series finale.
Pippa Mann, who climbed aided from her car, as well as JR Hildebrand have also been taken to the hospital by road.
Wheldon, this year's Indianapolis 500 winner, was one of at least four drivers whose cars became airborne in the crash, which occurred on the 11th lap of the race.
There is no information on the medical condition of Wheldon, but several other drivers involved in the accident have already been cleared from the Las Vegas Motorspeedway medical centre - including Tomas Scheckter, Paul Tracy and EJ Viso.
Other drivers involved in the accident, who climbed from their cars unaided were Jay Howard, Townsend Bell, Wade Cunningham, Vitor Meira, Charlie Kimble, Will Power, Alex Lloyd and Buddy Rice.
Tracy said of the accident upon leaving the medical centre: "Dan's car went over my back wheel. It was a horrendous accident. The medical guys are scrambling now, there are 20 doctors in the [the medical centre].
"A lot of young drivers were out there and they just don't know how to react in those situations."
Tony Kanaan, who was leading the race when it was halted, said: "One mistake can take 15 people out. I've never seen such a mess as I've seen today. It's going to be hard to keep your head straight. I think we need to rethink some things that we're doing on the racetrack. We have to take care of each other; we're playing with lives here."
The accident was triggered when Wade Cunningham made contact with James Hinchcliffe.
"The initial contact was with me. We were coming out of Turn 4 and I was side-by-side with Helio [Castroneves] and Graham [Rahal] and Ryan [Hunter-Reay] were banging wheels," said Hinchcliffe. "They got into each other and both had to lift and that allowed Wade, who was a couple of laps behind me to get a bit of a run and we went down the front straight three-wide.
"I was just looking at Helio's tyre trying to get as much room as high as I can and I guess going into Turn 1, Wade just got into my right-rear tyre with his front wing, which got his car loose and made him spin out in front of everybody else."