Traces of sleeping pills were found in the children's blood, likely used to "facilitate" their execution-style murder, said a statement issued by Nantes prosecutor Xavier Ronsin after drug tests were conducted on the victims.
French authorities have issued an arrest warrant for 50-year-old businessman Xavier Dupont de Ligonnes after the bodies of his wife and children, aged 13 to 20, were dug up in the garden of their family home in the western city of Nantes in Brittany.
Autopsies showed the victims had been "methodically" shot with a .22 calibre firearm similar to one that Dupont de Ligonnes had used in shooting practice at a Nantes rifle club.
Wife Agnes 49, daughter Anne, 16, and three sons were apparently shot several times in the head while asleep, around April 3 or 4.
Dupont de Ligonnes was last seen on April 15 when he left a budget hotel at Roquebrune-sur-Argens in the Var region on France's Mediterranean coast, abandoning one of the family cars in the car park.
Police have received about 500 witness accounts, but none has led to any clue of the man's whereabouts, Ronsin said in the statement.