Virgin Galactic “ignored” repeated warnings in the years leading up to the deadly crash of its spacecraft in California, a rocket science safety expert said Sunday, as investigators hunted for clues to the accident.

Carolynne Campbell, a rocket propulsion expert with the Netherlands-based International Association for the Advancement of Space Safety, said she could not speculate on the cause of Friday’s crash without “all the data.”

However, she said multiple warnings about the spacecraft’s motor and the fuel used to power it had been issued to Virgin since 2007, when three engineers died testing a rocket on the ground.

“Based on the work we’ve done, including me writing a paper on the handling of nitrous oxide, we were concerned about what was going on at Virgin Galactic,” she told AFP.

“I sent copies of the paper to various people at Virgin Galactic in 2009, and they were ignored.”

Campbell said she outlined concerns to Virgin Galactic in a subsequent telephone conversation, but her warning again went unheeded.

“I warned them… that the rocket motor was potentially dangerous,” she said.

Campbell’s warnings related to nitrous oxide, reportedly used as a fuel component in the doomed craft along with a new substance derived from nylon plastic grains.

A team of investigators from the National Transportation Safety Board (NTSB) has been deployed to the Mojave Desert to probe Friday’s crash, which left pilot Michael Alsbury dead and and co-pilot Pete Siebold seriously injured.

– ‘Safety number one priority’ –

British tycoon Richard Branson said that safety had always been Virgin’s paramount concern.

“Safety has always been our number one priority,” Branson said, adding that the company would not “push on blindly” with its ambitious space program until the causes of the accident had been determined.

Branson, however, took aim at early speculation of the causes of the crash, which had focused on the new rocket fuel.

“To be honest, I find it slightly irresponsible that people who know nothing about what they’re saying can be saying things before the NTSB makes their comments,” he told reporters in Mojave on Saturday.

Virgin Galactic chief executive George Whitesides also questioned the safety claims, telling the Financial Times in an interview published Sunday that differences of opinion were common in the world of space flight development.

“In the space community, you will be able to find people who have favorite technologies of different types. One group will say their type of technology is better than another,” the paper quoted him as saying.

“We pay a lot of attention to the several hundred engineers that we have on staff, and other expert consultants we’ve talked with about our technologies.”