A retired couple was found dead at their home in Notre-Dame-de-Riez (Vendée) late Sunday afternoon.
The criminal trail is privileged after the son has been taken into custody.
The two victims did not die by firearm, autopsies will be performed in the coming days to determine the cause of death.
The bodies were discovered overnight from Saturday to Sunday in the house where the retired couple lived with their son, born in 1984.
According to Ouest-France, the firefighters arrived during the night.
Residents were asked to leave the area from 6 a.m., and could not return until 4 p.m.
The gendarmes were still working on the scene at the end of the day.
"The gendarmes took precautions to secure the scene", detailed in particular to the regional daily the mayor of the town, Hervé Bessonnet, called to the scene in the morning.
"It's dramatic," he still reacted.