Neighbours and a source close to the probe had said that Sebastien had suffered from psychological problems and Pascal from depression in the past.
Bloodstains were found throughout the two-storey house, including on Sebastien's cellphone and on Brigitte's watch, as well as efforts to wipe some of them away. No toothbrushes or hairbrushes were found in the house, and the beds had been stripped, with some sheets drying on an indoor rack.
Food was going off in the refrigerator and there were dishes in the sink. The four family members' bank accounts had remained inactive, and their cellphones had not been activated since February 18.
Sebastien's cellphone was the last to be switched off.
"It's as if the life of the house was frozen in time," had said Pierre Sennès, Nantes prosecutor.
With police focusing on the son, who had a previous conviction for making death threats on his blog, attentions suddenly turned to Pascal's sister and her husband, who were arrested over the weekend.
The estranged brother-in-law, Hubert Caouissin, 46, a construction engineer and Pascal Troadec's sister, Lydie, 47, initially denied involvement, although he mentioned "tensions" with Troadec who he said he hadn't seen "in years".
But police detained the pair yesterday after Caouissin's DNA was found on a glass in the Troadec home and Sebastien's car, recovered last Friday in a car park. Personal belongings of the daughter and father were later found near Plouguerneau, near Brest, where the sister and her husband live.
Confronted with the damning evidence, the brother-in-law confessed to the murder of all four, the prosecutor confirmed.
He told police that he had killed the four with a crowbar and then dismembered the bodies and burnt them in another part of Brittany.
The sister, however, is refusing to talk to investigators.
In a press conference, Sennès said the motive was an "old family conflict" over an unspecified number of gold coins that the sister and her husband accused Troaedec of inheriting from his father and failing to share with them.
But the gold may well never have existed. Police have found no evidence of the coins.
Sennès said Caouissin had "spied" on the family on the evening of February 17, listening to their conversations through the wall via a stethoscope.
Once they were all asleep, he crept into the house via the laundry room looking for a key he had spied from outside. But he made a noise, waking the Troaedec parents who went to confront the intruder, Pascal armed with a crowbar.
"A fight broke out between the two men and Huber Caouissin managed to seized the crowbar", using it to bludgeon the couple to death, said Sennès. It is thought he used the same blunt instrument to kill the children, he added.
He stayed in the house until early morning, and then returned home and confessed to his wife the terrible events.
The following night, he returned to the murder scene, cleaned the premises of blood and bundled the bodies into Sébastien's car boot.
"It appears that the bodies were dismembered and burned and buried" in Brittany, said Sennès.
Caouissin's mother, Evelyne, told Le Parisien that the sister and her husband had fallen out over the treasure's alleged existence, which they claimed Pascal's father had found hidden in the walls of a flat he had bought The row poisoned relations, and contributed to Caouissin's suffering professional "burn out", she said.
Neighbours said that the couple had "gone through a difficult time, particularly financially" and became obsessed with their more moneyed family members.
"There was jealousy. They had the impression they came across as losers while Pascal and Brigitte made the most out of life. They seemed to take it badly," a source close to the inquiry told Le Monde.
The pair have been remanded in custody awaiting charges.
