The bodies of a 42-year-old university professor, his wife, 38, son, 18, and mother-in-law, 63, were found Thursday in the basement in the town of Laishevo in the Russian republic of Tatarstan, RIA Novosti reported.
The victims apparently died in successive stages as they went into the basement to rescue those who had gone before, investigators said.
The man is believed to have fainted after he went into the basement. His wife and son then went down and succumbed to the gas. The mother-in-law called a neighbor for help before going into the basement.
"They likely died through poisoning by the gas that had built up in the basement from rotting potatoes," said a statement by the local investigative committee.