RIO DE JANEIRO: At least six people died on Sunday after two helicopters collided mid-air in western Rio de Janeiro, firefighters said. American singer-songwriter Oliver Tree, who was in Brazil on a world tour, was aboard one of the helicopters that crashed, leaving all occupants dead, a police source told AFP. The alternative singer and internet personality is listed as one of six people on the manifests of the two aircraft shared by the source, who said the victims could not be formally identified yet as they were badly burned in the crash.