Four members of a French family died after jumping from a seventh-floor apartment in the Swiss town of Montreux, police said. A 15-year-old boy survived the tragedy last Thursday near the casino on Lake Geneva. He remains in a coma in a stable condition in hospital. The Vaud regional police said on Tuesday there was no sign of a struggle and that their findings “make it possible to rule out the intervention of a third party”. Police and prosecutors are working on the theory of “collective suicide”. A man aged 40, his 41-year-old wife, her twin sister, the couple’s eight-year-old daughter and their son all lived “withdrawn from society”, according to police. Investigators said two officers knocked on the apartment door at 6.15am, wanting to speak with the father about home-schooling arrangements for his son. Unable to enter, the officers left. The police said that “since the start of the pandemic, the family was very interested in conspiracy and survivalist theories”. The family lived in virtual self-sufficiency having amassed a stockpile of food, taking up much of their living space but enabling them to see out a major crisis. Only the mother’s twin sister worked outside the home, while neither the mother nor the eight-year-old girl, who did not attend school, were registered with the local authorities. “All these elements suggest … fear of the authorities interfering in their lives,” the police statement said. France’s Journal du Dimanche newspaper said the father, Eric David, grew up in a wealthy part of Marseille and attended the Ecole Polytechnique, one of the most prestigious schools in the country. The twin sisters, Nasrine and Narjisse Feraoun, grew up in a family of five children who were all educated at the elite Lycée Henri-IV in Paris, the weekly said. The mother was a dentist and her sister an ophthalmologist. The newspaper also said the twins were granddaughters of Algerian novelist Mouloud Feraoun. A close friend of the French philosopher Albert Camus, Feraoun was assassinated in Algiers in 1962 by a far-right French pro-colonial group. In the UK and Ireland, Samaritans can be contacted on 116 123 or email jo@samaritans.org or jo@samaritans.ie. In the US, the National Suicide Prevention Lifeline is 1-800-273-8255. In Australia, the crisis support service Lifeline is 13 11 14. Other international helplines can be found at befrienders.org