'Buckwild' Star Shain Gandee -- Carbon Monoxide Poisoning Possible Cause of Death

2:38 PM PT -- TMZ has learned ... the bodies of all 3 victims have been recovered from the scene and have been transferred to the local medical examiner's office where autopsies will be performed.

2:29 PM PT -- Officials have retrieved Shain's red Ford Bronco from the muddy pit where it was discovered ... and judging by pictures of the car taken at the scene, it appears the vehicle was submerged past the window.

The photos seem to support the theory that the passengers died from carbon monoxide poisoning because the deadly fumes from the car could not escape through the tailpipe, which was completely submerged in the mud.

There is evidence that "Buckwild" star Shain Gandee may have died from carbon monoxide poisoning ... sources close to the death investigation tell TMZ.

A rep for the Kanawha County Sheriff's Office in West Virginia tells us, the bodies of Shain, his uncle David Gandee, and a third unidentified person are still inside the truck where they were originally found. Authorities have been unable to remove the bodies from the scene because of the truck's location.

According to the rep, Shain's 1984 Bronco is stuck knee-deep in mud in the middle of nowhere ... and the only way to reach it is via an all-terrain vehicle.

We're told there is no cell reception in the area, which is why Shain and the others couldn't be reached. We're also told the bodies were originally discovered today by a passing ATV rider.

Police wouldn't confirm or deny whether carbon monoxide poisoning played a role in the deaths -- but sources connected with the investigation say it's the likely scenario, especially if the exhaust pipe was submerged in mud.