Gandee's SUV was stuck in a mud pit near his home in Sissonville, its tail pipe submerged. That could have allowed the invisible gas to fill the vehicle's cabin.
Gandee, his 48-year-old uncle, David Gandee, and 27-year-old friend Donald Robert Myers had last been seen leaving a bar at 3am March 31. Friends and family searched all day for them, and authorities issued a missing-persons report the following day.
The Gandees were buried Sunday after a joint memorial service in Charleston that drew hundreds of friends, family and fans. Cameras were not allowed inside the Charleston Municipal Auditorium or at the private family burial in Thaxton Cemetery.
Shain Gandee, nicknamed "Gandee Candy" by fans, was a breakout star of the show that followed the antics of young friends enjoying their wild country lifestyle.
Season one was filmed last year, mostly around Sissonville and Charleston.
Gandee favoured four-wheelers, pickups and SUVs over cellphones and computers, and "mudding," or off-road driving, was one of his favorite pastimes. It was no coincidence some mourners arrived in mud-splattered trucks.
Shooting was underway on season two at the time of Gandee's death, but MTV said film crews were not with him over Easter weekend and hadn't filmed him since earlier that week.
MTV said the half-hour series in the old Jersey Shore time slot was pulling in an average of 3 million viewers per episode since its premiere and was the No. 1 original cable series on Thursday nights among 12- to 34-year-olds.
Some fans reacted angrily to the decision on social media, launching a campaign to keep the show alive using the hash tag (hash)KeepBuckwildForShain on Twitter. Cast member Cara Parrish was also among those objecting.
"I think MTV should show reality. Losing Shain broke all our hearts," Parrish tweeted.
Although the idea of filming without Gandee "hurts me clear through my soul," she wrote, "the thought of turning our backs on his dreams is worse".
But many others, including some at Gandee's funeral, said the show just wouldn't have been the same without him.
The network issued a statement from Loretta and Dale Gandee, thanking fans for their thoughts and prayers.
"We have truly felt all the love and know that Shain is resting peacefully," they said.
"Shain was an incredible, outgoing and positive person who was loved by whoever he met. We are honored that we were able to let the world see what a wonderful son we had."
Shain Gandee was the third BUCKWILD cast member to make unwanted headlines.
Last month, 24-year-old Salwa Amin was sent back to jail for violating the terms of her bond following a February arrest on drug charges. She is facing two counts of drug possession with intent to deliver and remained behind bars without bond on Wednesday.
State Police say a multi-agency task force arrested Amin and two other people at a Summersville residence after receiving a tip from an informant. A search found oxycodone pills, heroin and $3,000 in cash.
Another cast mate, Michael Douglas Burford, was charged in February with driving under the influence.
Some detractors, including US Senator and former West Virginia Governor Joe Manchin, had publicly worried about the show and its cast before the first episode aired.
Manchin asked MTV to cancel the show, telling the network's president that it would profit from the "poor decisions of our youth" and that it played to ugly, inaccurate stereotypes about the people of West Virginia.
