Investigators are continuing to probe what caused a helicopter to plunge into the Grand Canyon as it emerged that the crash happened on tribal land where air tours are not as highly regulated as those inside the national park itself. Three British tourists died when the helicopter went down on Saturday. They were in Las Vegas to celebrate a birthday and took a helicopter sightseeing tour on the Hualapai reservation. Becky Dobson, 27, her boyfriend Stuart Hill, 30, and his brother, Jason Hill, 32-year-old lawyer died. Three friends and the pilot are in hospital where investigators want to interview them about what went wrong. Unlike the national park, air tours on the Hualapai reservation are not subject to federal regulations that restrict routes, impose curfews and cap the amount of flights over the Grand Canyon each year. The Federal Aviation Administration granted the Hualapai Tribe an exemption 18 years ago after finding that the regulations would harm the tribe's economy where tourism is a major driver. Most of the flights over the reservation originate from Las Vegas, and air tour operators aggressively market them. The pilots can fly between canyon walls and land at the bottom next to the Colorado River on the reservation, which isn't allowed at the park other than for emergency operations. Quartermaster Canyon locator The National Transportation Safety board says it can't say with any certainty yet what caused Saturday's crash. Gary Robb, an attorney who has represented crash victims for almost 40 years, told the Las Vegas Review-Journal it may take investigators as long as nine months to establish the cause of Saturday’s crash. “It’s too early to speculate, but early indications suggest that perhaps heavy gusts could have been a factor that drove the aircraft to strike a wall of the canyon,” he said. “The other possibility is some sort of in-flight mechanical issue, including an engine problem or main rotor blade fracture or defect. Stuart Hill and Becky Dobson died when a helicopter crashed in the Grand Canyon on Saturday Credit: facebook “You also cannot rule out human error, whether it be some sort of pilot incapacitation or neglect.” Papillon Grand Canyon Helicopters said it is co-operating with the investigation and that it abides by flight safety rules and regulations that exceed those required by the Federal Aviation Administration. Saturday’s crash was its fourth fatal accident in the past 20 years, claiming eight lives.