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

West Virginia police have just confirmed what TMZ reported yesterday ... "Buckwild" star Shain Gandee died from carbon monoxide poisoning.

A rep for the Kanawha County Sheriff's Office released a statement saying Shain's autopsy was completed today. The death was ruled an accident.

Shain's uncle and a male friend also died while inside Shain's 1984 Bronco, which had stalled out in mud while the crew went off-roading over the weekend. The truck's exhaust pipe had apparently been submerged in mud, causing the gas to leak back into the car.

As we previously reported ... the 21-year-old reality show star as well as the two others were found dead yesterday morning by a passing ATV rider.