Jeremy Renner was trying to stop a six-tonne snowplough from crashing into his nephew when he became trapped and crushed by the machine, according to a police report.
The Marvel actor, 52, was using the PistenBully plough to help Alexander Fries free his car from snow when the large vehicle began to roll down a hill.
Renner, known for playing superhero Hawkeye, sustained injuries to his “torso, face, extremities and head”, the report from Washoe county sheriff’s department said.
The actor previously said he had broken more than 30 bones, but they would “grow stronger, just like the love and bond with family and friends deepens”.
The incident took place around the new year, near the Mount Rose Highway, which links Lake Tahoe and south Reno as it straddles the Nevada-California border in the US.
A report by deputy Garret Leone, obtained by the Press Association, said that after he had arrived at the scene of the accident, he had spoken to Fries – who had been the only one present.
“Jeremy is his uncle and was helping him get the Ford pickup truck out of the driveway.
"The pickup truck got stuck multiple times, so they used the PistenBully to pull the truck out of the driveway,” the report said.
“They had a chain connected to the back of the PistenBully and to the front of the Ford pickup.
"They got the pickup truck out of the driveway and Alexander parked in the middle of the street.
“Jeremy turned the PistenBully around further west on Drive. Once turned around, Jeremy exited the PistenBully in an attempt to speak to Alexander.
“Alexander observed the PistenBully traveling eastbound on Drive directly towards him with nobody inside driving the vehicle.
“While running towards the truck, Alexander observed Jeremy jumping on the tracks of the PistenBully to get in the cab of the vehicle.
"Alexander said Jeremy got caught on the PistenBully trucks and was ultimately run over by the PistenBully.”
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Fries had begun “screaming for help” as he did not have his phone and was helped by two neighbours who provided towels and assistance to Renner while medical personnel arrived.
Another report from deputy Jonathan Miller added that “in an attempt to keep the PistenBully from striking Fries, (Renner) attempted to step up on the track in order to divert the PistenBully or get it stopped.
“Renner was pulled under the PistenBully, and the track rolled over him,” the report said.
The actor has kept fans updated with his recovery process and on Saturday thanked those who sent him messages of support.
Posting a photo on Instagram of him receiving treatment while lying in a hospital bed he wrote: “Morning workouts, resolutions all changed this particular new years…
“Spawned from tragedy for my entire family, and quickly focused into uniting actionable love.
“I want to thank EVERYONE for their messages and thoughtfulness for my family and I. Much love and appreciation to you all.
“These 30 plus broken bones will mend, grow stronger, just like the love and bond with family and friends deepens. Love and blessings to you all.”