People, including a boy (front, 3rd R) holding the French national flag, take part in a vigil to pay homage to the victims of the helicopter crash in Villa Castelli March 12, 2015. French and Argentine investigators scoured the charred wreckage of two helicopters in the remote western Argentina province of La Rioja on Thursday for clues to what caused an in-flight collision that killed 10 people, including three French sports stars. The head of Argentina's investigation team promised a meticulous investigation into Monday evening's accident, which killed Olympic swimmer Camille Muffat, yachtswoman Florence Arthaud and boxer Alexis Vastine. REUTERS/Marcos Brindicci (ARGENTINA - Tags: DISASTER SPORT TRANSPORT OLYMPICS)