They have arrested a 35-year-old jobbing builder, Karim Ben Salah, on suspicion of murdering two Dutch couples at their holiday retreat in the village of Montfort, west of Toulouse, last month.
Mr Ben Salah, who was working at the house that day, denies any part in the crime. An investigating judge has placed him under formal investigation - a step short of a formal charge - for "murders, preceded by acts of barbarism, theft and attempted swindling".
The bodies of Artie Van Hulst, 51, a Dutch businessman, his wife Marianne, also 51, and her sister and brother-in-law, Dora and Johan Nieuwenhuis, both 62, were found at the Van Hulsts' holiday home on 24 May. Mr Van Hulst was shot four times by a rifle at close range. The others were tied up and stabbed to death.