FLAGSTAFF, Ariz. ó A helicopter crash that killed three British tourists and left four others critically injured happened on tribal land in the Grand Canyon where air tours are not as highly regulated as those inside the national park. The group of friends was in Las Vegas to celebrate a birthday and took a helicopter sightseeing tour of the Grand Canyon on the Hualapai reservation, family and friends said. Those killed were veterinary receptionist Becky Dobson, 27; her boyfriend, car salesman Stuart Hill, 30; and his brother, Jason Hill, a 32-year-old lawyer. Unlike the national park, air tours on the Hualapai reservation are not subject to federal regulations that restrict routes, impose curfews and cap the number of flights over the Grand Canyon each year. The Federal Aviation Administration granted the Hualapai Tribe an exemption nearly two decades ago after finding that the regulations would harm the tribeís economy, in which 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 isnít allowed at the park other than for emergency operations. Landing pads sit upstream and downstream from where the copter owned by Papillon Grand Canyon Helicopters crashed Saturday, constantly ferrying people on and off aircraft. The National Transportation Safety Board is investigating what led to the crash in a remote area where rescuers had to fly in, hike to the site and use night-vision goggles, Hualapai Nation police Chief Francis Bradley said. Windy conditions and the rugged terrain made it difficult to reach the wreckage. The NTSB wonít say with any certainty what caused the crash until its investigative report is released, which could take up to two years. Papillon said that it is cooperating with the investigation and that it abides by flight safety rules that exceed FAA requirements. A company spokeswoman did not respond to requests for more information Monday. Aviation lawyer Gary C. Robb said potential factors were winds of 10 mph with gusts of 20 mph, pilot error, mechanical failure or company pressure to meet the demand for tours. "You can replace a helicopter. You canít replace those three lives that were lost," he said.Flights into the canyon outside the national park were restricted Monday. Investigators hope to speak to the four survivors as they begin to recover from critical injuries, Stein said. The pilot, 42-year-old Scott Booth, suffered a limb injury, tribal police said. The other survivors from the United Kingdom are Ellie Milward, 29; Jonathan Udall, 32; and Jennifer Barham, 39. Dobsonís father, Peter, told Britainís Press Association news agency that his daughter and Stuart Hill "were really happy together" and they were celebrating his 30th birthday with friends. "They were always going out and doing things, just enjoyed being with each other," he said. "The whole thing is just terrible." The brothersí father, the Rev. David Hill, said his sons were "incredibly close."