Six British friends boarded a sightseeing helicopter outside Las Vegas on their way to one of the busiest and noisiest stretches of the Grand Canyon -- tribal land that is less regulated than the national park and allows tour pilots to descend into the natural wonder.
The group never came back from the Hualapai reservation in northwestern Arizona, where the crumpled wreckage of the EC-130 helicopter lay smoldering in the darkness of the rugged gorge.
Three British tourists died and four others were critically injured Saturday, leaving federal investigators to determine what led the group's birthday celebration to end in tragedy.
Aviation attorney Gary C. Robb said potential factors could include winds gusting to an estimated 20 mph, pilot error, mechanical failure or pressure on Papillon Grand Canyon Helicopters to meet demand for the popular Grand Canyon air tours to the reservation.
Investigators are likely to pay special attention to Papillon's EC-130 helicopter, which generally lacks a system to keep it from exploding on impact, denying passengers a few extra minutes to try to escape, Robb said.
Witnesses described a "horrible" scene of flames, smoke and small explosions.
Unlike the more tightly regulated air tours within Grand Canyon National Park, helicopters on tribal land function like an airborne shuttle service, quickly depositing tourists on landing pads for lunches or hikes at the river's edge or pontoon boat rides. Just as quickly, they whisk them away.
The Federal Aviation Administration granted the Hualapai Tribe an exemption nearly two decades ago to regulations on flight times, routes and total trips because the rules would harm the tribe's economy, which relies on the busy flow of tourists.
Papillon Grand Canyon Helicopters said it is cooperating with the investigation and abides by flight safety regulations that exceed those required by the FAA.
Hualapai tribal leaders said they are halting helicopter tours at the canyon for now and are working with federal investigators to "find out exactly what happened here, in what marks the first such incident to claim the life of a passenger at the West Rim in 15 years."
The victims are veterinary receptionist Becky Dobson, 27; her boyfriend Stuart Hill, a 30-year-old car salesman; and his brother, Jason Hill, a 32-year-old lawyer.
An Arizona medical examiner said their autopsies were expected to be completed by Tuesday afternoon.
Police said pilot Scott Booth, 42, severely injured a limb. The other survivors being treated for critical injuries at a Las Vegas trauma center are Ellie Milward, 29; Jonathan Udall, 32; and Jennifer Barham, 39.
A witness saw two badly injured women, one who appeared burned over most of her body and the other covered in blood.
"It's unimaginable, the pain," wedding photographer Teddy Fujimoto said.
Dobson's father, Peter, told Britain's Press Association news agency that his daughter and Stuart Hill "were really happy together" and they were celebrating his 30th birthday with a group of friends.
"They were always going out and doing things, just enjoyed being with each other," he said. "The whole thing is just terrible."
The brothers' father, the Rev. David Hill, said his sons were "incredibly close.
"The two brothers loved each other and were very close, and so our misfortune is their support -- because they went together, and I will thank God every day for them," he said.