"The investigation is considering several versions of what happened: cotton ... could have happened due to poor quality gas supply services or careless actions associated with improper operation of gas equipment," the Russian IC said in a statement.
Two victims of the explosion in a residential building in Naberezhnye Chelny are in serious condition.
Earlier it was reported that the number of victims of the explosion in a residential building in Naberezhnye Chelny increased to five.
Later, one of the victims died in the hospital.