Paris - Investigators have widened a corruption probe into the French assets of three African ruling families, charging the daughter and son-in-law of Congo's President Denis Sassou Nguesso, judicial sources told AFP on Sunday.Julienne Sassou Nguesso, 50, and her 53-year-old husband Guy Johnson were placed under investigation this week for "money laundering and misuse of public funds", the sources said.Investigators are trying to determine how the couple in 2006 were able to purchase a mansion valued at $3.4 million in the swanky Paris suburb of Neuilly-sur-Seine just north of the ritzy 16th arrondissement, according to a judicial source. The tentacles of the case also reach out to ruling families in Equatorial Guinea and Gabon.Julienne Sassou Nguesso is an insurance agent by profession and her husband is a lawyer.Offshore accounts Between 2007 and 2011, the seven-bedroom house with an indoor pool underwent renovations of 5.34 million euros, raising the couple's total investment to nearly 10 million euros.Investigators believe the couple may have financed part of the project through an offshore company in the Seychelles and with the sale of shares Julienne Sassou Nguesso owned in a telecommunications company tied to "corruption operations", the source said.The French finance ministry's intelligence unit Tracfin alerted the authorities to possible misconduct.Investigators then found that millions of euros of state money had been funnelled from Brazzaville since 2007 to offshore accounts in the Seychelles, on Mauritius Island and in Hong Kong, which they believe were used to fund a lavish lifestyle for members of the presidential family."This affair that has been going on for 10 years will be dismissed through totally legal procedures," Jean-Marie Viala, Denis Sassou Nguesso's lawyer, told AFP. A blind eye Johnson is also under investigation for the role he may have played as the asset manager of a real estate company that in 2007 bought a mansion worth 19 million euros in another posh area of Paris.Denis Sassou Nguesso's oldest daughter Edith Lucie Bongo Ondimba, who died in 2009, also owned shares in that company, as well as Gabon's late president Omar Bongo, who was married to Ondimba. French investigators have been looking into the entire Sassou Nguesso clan as well as relatives of Omar Bongo and his son, the current ruler, and President Teodoro Obiang Nguema of Equatorial Guinea.Obiang's son, Teodorin, is currently fighting a case in a Paris court for having allegedly pillaged the country's assets to fund a lavish personal lifestyle.The 48-year-old, known for his taste for supercars, luxury homes and bespoke suits, is suspected of using more than $112 million of state money - proceeds of corruption and embezzlement, prosecutors allege - to buy a six-storey mansion on Avenue Foch, one of the swankiest streets in Paris, as well as a collection of Italian supercars.African dictators "At a time when we're nearing the outcome of the Obiang case, we're getting closer to the opening of a trial of the Sassou Nguesso clan," said William Bourdon, a lawyer for the anti-corruption NGO Transparency International.Obiang has denied the charges, saying the money came from legitimate sources.The trial set a precedent for France which has long turned a blind eye to African dictators parking ill-gotten gains in Parisian real estate and luxury products.It came about after nearly a decade of lobbying by activist groups Sherpa and Transparency. Already in March, the nephew of the Congo-Brazzaville president, Wilfrid Nguesso, was placed under investigation on the same counts.The development is part of a series of enquiries by the French authorities into the assets of three African presidential families that began in 2010.