According to a statement by Saint Gallen cantonal police, Swiss child welfare office APEA had already filed a complaint against the parents on suspicion they had violated the welfare and education obligations towards their child, whose whereabouts could not be determined.
The public prosecutor had ordered an investigation, which led police to discover a body in the basement of the house where the girl lived.
Her parents, a 32-year-old German woman and a 52-year-old Swiss man, were subsequently arrested.
According to the statement, the state of the body means official identification has not yet been confirmed, but investigators are working on the assumption that it is the body of the missing child. Her age has not been released.
The cause of death is not yet known.