Jeremy Renner suffers extensive injuries after Snowcat runs over his leg.
Neighbor, who is a doctor, applies a tourniquet to stop the bleeding.
Soon after, Renner was airlifted to the hospital in critical but stable condition.
Police impound Snowcat to investigate the incident.
"Extensive" safety features were in place, but the machine still ran over Renner.
First responders are concerned about access to the accident sites due to weather.
Renner recently tweeted about heavy snowfall in Lake Tahoe.
Fans are sending well wishes for Renner's speedy recovery.