The gruesome discoveries began with a severed leg being dug up at the house in Nantes.
Then officers found the corpses, which had bullet wounds.
Officials say the bodies are believed to be those of Agnes Dupont de Ligonnes, 49, and her four children: Arthur, 21; Thomas, 18; Anne, 16; and Benoit, 13.
Xavier Dupont de Ligonnes, the father, remains missing and there are no signs of a struggle in the house.
The family disappeared on April 3 or 4. Police believe they were kidnapped and killed. Their two dogs were also found buried.
The case has been complicated further by strange notes and emails obtained by officers.
One letter suggests the family wanted to move to Australia, while an email sent by the father said he was a secret US agent about to testify in a drug case, and leaving to join a witness protection programme.
Post-mortem examinations are set to take place on the bodies on Friday. Source