PARIS (AFP) - The brother-in-law of a Frenchman missing along with his wife and two children for over a fortnight has admitted killing all four of the family, sources close to the probe said Monday (March 6).

Pascal Troadec and his wife Brigitte, both 49, and their 21-year-old son Sebastien and daughter Charlotte, 18, have been missing on Feb 16 in a case that has gripped France.

Police searches at the couple's suburban home in the western city of Nantes found traces of blood from Sebastien and the parents, but not Charlotte, as well as signs of efforts to clean them up.

Pascal Troadec's sister and her husband - already quizzed by investigators at the start of the probe - were taken into custody on Sunday (March 5).

One investigation source said on Monday that the brother-in-law, who has not been named, had admitted killing the family in a row over an inheritance.

The Daily Mail reported that Mr Troadec had inherited gold coins and ingots, which are said said to have been handed down to him by his late father, who died six years ago.

The 46-year-old brother-in-law, identified as Hubert C by The Daily Mail, is understood to have felt his 47-year-old wife had been wronged by not getting a fair share of the gold.

He is said to have slaughtered the Troadec family in their detached home in Nantes, western France, during a row overt the ingots and coins.

It has been claimed that he battered them to death with a blunt object, and then tried to cover up the evidence.

Initial suspicion fell on the son, as Sebastien was suspected of "formulating a macabre plan aimed at snuffing out members of his family and maybe himself".

According to The Daily Mail, neighbours and a source close to the probe said Sebastien had had psychological problems and Mr Troadec had suffered from depression in the past.