Carolynne Campbell, a rocket propulsion expert with the Netherlands-based International Association for the Advancement of Space Safety (IAASS), 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 reporters.
"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.
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.