The bodies were found Wednesday, after a woman noticed the vehicle on her way to work, Winston County Sheriff Rick Harris said.
Authorities are investigating the deaths, WBRC-TV, Birmingham, Ala., reported.
The vehicle was found off a narrow road that is difficult to navigate.
Harris said there is potential evidence around the vehicle, and the four victims, who were described as severely injured, may have been shot to death.
"Right now, we have more questions than answers," he said.
The vehicle is from out-of-state, authorities said.
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