The investigation into last week's deadly crash of the Virgin Galactic spaceship has found that the "feathering" function to help it descend into the atmosphere from space was deployed early, the National Transportation Safety Board said on Sunday.
SpaceShipTwo's rotating tail boom, a key safety feature for re-entering the atmosphere, inadvertently rotated early, said Christopher Hart, the acting chairman of the NTSB.
The federal agency is leading the investigation into what caused the spacecraft to crash in California's Mojave Desert during a test flight on Friday, killing one pilot and badly injuring the other.
Virgin Galactic is the fledgling space tourism company of billionaire entrepreneur Richard Branson.
Hart told a news conference late on Sunday investigators had determined the "feathering" system should have been deployed when the vehicle was travelling about 1.4 times the speed of sound. Instead, the feather began rotating when the vehicle was travelling at Mach 1, he said.
The system, which folds the vehicle in half to create more atmospheric drag, was unlocked early by the co-pilot but a second command to move the feather handle into position was not sent, he said.
Investigators also recovered SpaceShipTwo's propellant tanks and engine intact, indicating there was no explosion.
"The engine burn was normal up until the extension of the feathers," Hart told presspersons.
He emphasised that the NTSB was not saying the early deployment of the ship's feather was the cause of the accident and stressed that the investigation was in its earliest stages.
"We have months and months of investigation," Hart said.
SpaceShipTwo was conducting test flights and was not yet certified for commercial operations when the crash occurred, delaying indefinitely the start of passenger service.
Branson and his son plan to fly on the first commercial flight. About 800 people already have paid or put down deposits for the ride, which costs $250,000.
The two pilots involved were employees of Scaled Composites, a Northrop Grumman Corp subsidiary that designed and built the six-passenger, two-pilot craft for Virgin Galactic.
Michael Alsbury, 39, has been identified as the co-pilot who died. Peter Siebold, the 43-year-old pilot riding in the right-hand seat, parachuted to the ground and was recovering at a nearby hospital, Scaled Composites said in a statement.
Friday's crash was the second disaster in less than a week suffered by a private space company.
On Tuesday, an Antares rocket built and launched by Orbital Sciences Corp exploded after liftoff from Wallops Island, Virginia, destroying a cargo ship bound for the International Space Station.
Virgin Galactic is a US offshoot of the London-based Virgin Group founded by Branson, whose empire ranges from airlines to music stores and mobiles phones.