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The three British tourists who died in a helicopter crash at the Grand Canyon on Saturday were there to celebrate the 30th birthday of one of the passengers, relatives said Monday.

Speaking to the British Press Association, Peter Dobson said his daughter, Becky, was there for a long weekend to celebrate the birthday of her boyfriend, Stuart Hill.

“They had been looking forward to it for a long time,” Dobson said, adding: “The whole thing is just terrible."

Stuart and his brother Jason also died on the trip, according to their father, Reverend David Hill, who spoke with The Evening Standard in London.

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“They were incredibly close, and as parents we feel blessed to have had them, but a light has truly gone out,” David Hill told the paper, adding that the group had saved for a year to take the trip.

Jason's employer, the Shoosmiths law firm, said in a statement to NBC News that he'd worked in the firm's corporate division since 2011.

Three other passengers on the aircraft, along with the pilot, were injured but survived, Hualapai Nation Police Chief Francis E. Bradley Sr. said during a news conference Sunday, adding that the passengers were all from the United Kingdom.

The helicopter, a Eurocopter EC130, took off from the Las Vegas area and crashed at roughly 5:30 p.m. (7:30 p.m. ET) near Quartermaster Canyon in the Grandy Canyon West area.

Stephen Stein, an investigator with the National Transportation Safety Board, said Monday it was unclear what caused the crash.

Asked during a news conference if the aircraft had a crash resistant fuel system — which prevents quick explosions after impact, allowing survivors to flee wreckage — Stein said that was an “important” part of the probe that would be determined later.

This photo shows the scene of a helicopter crash at the Grand Canyon on Saturday, Feb. 10, 2018. Authorities said three people were killed and four others were injured. Courtesy of Teddy Fujimoto

Images from the scene showed the aircraft on fire and a witness, Teddy Fujimoto, told the Associated Press that one of the survivors was badly burned.

A helicopter crash lawyer, Gary C. Robb, told NBC News that the Federal Aviation Administration has required crash resistant fuel systems since 1994 though many registered aircraft don’t have them.

“If you can buy 30 seconds, if you can buy a minute, that’s the difference between getting out of the helicopter unscathed and being a horrific burn victim,” Robb said.

An FAA spokesman, Allen Kenitzer, couldn’t immediately say how many FAA-registered helicopters have the systems.

A spokeswoman with Papillion Group, the aircraft operator, did not respond to requests for comment, but in a prepared statement, CEO Brenda Halvorson said it was cooperating with NTSB investigators.