Investigators looking into a deadly tour bus crash on a highway near the Hoover Dam said Saturday that it may take up to a month to figure out why the vehicle overturned, killing seven Chinese tourists and injuring 10 others. Arizona Department of Public Safety Sgt. Danny Hannigan said the National Transportation Safety Board was helping to investigate the cause of Friday's crash. The bus had been taken to Kingman where investigators will scour it over the next week, department spokesman Lt. James Warriner said. Six people were pronounced dead at the scene and a seventh died at a hospital, said Cmdr. Dean Nyhart of the public safety department. The driver was among those in serious condition, he said.