An investigation into the deadly crash of a Virgin Galactic spaceship has found that a function to help the craft descend into the atmosphere was deployed early, a federal safety official said on Sunday, adding pilot error could not be ruled out.

The National Transportation Safety Board (NTSB) 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.

SpaceShipTwo’s rotating tail boom, a key safety feature for re-entering the atmosphere, rotated early, Christopher Hart, acting chairman of the NTSB, said late on Sunday, though he said it was too early to say whether this had caused the crash.

Virgin Galactic is the fledgling space tourism company of billionaire entrepreneur Richard Branson.

Hart told a news conference that investigators had determined the “feathering” system, which helps it descend into the atmosphere from space, should have been deployed when the vehicle was traveling about 1.4 times the speed of sound.

Instead, the feather began rotating when the rocket-powered vehicle was traveling at Mach 1, he said, using a technical term for the speed of sound at a given altitude.

Hart said the feathering system, which folds the vehicle in half to create more atmospheric drag, was unlocked early by the co-pilot, according to video from the spaceship’s cockpit. About two seconds later, the spaceplane’s tail section began to fold.

“I’m not stating that this is the cause of the mishap. We have months and months of investigation to determine what the cause was,” Hart said.

Asked if the NTSB was considering the possibility of pilot error, Hart said: “We are not ruling anything out. We are looking at all of these issues to determine what was the root cause of this mishap  We are looking at a number of possibilities, including that possibility [of pilot error].”

No Explosion

SpaceShipTwo was released normally from its carrier aircraft, WhiteKnightTwo, at an altitude of about 45,000 feet. Shortly afterwards its hybrid rocket motor, which was flying for the first time with a new plastic propellant, ignited as planned, Hart 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 reporters.

SpaceShipTwo was conducting test flights and was not yet certified for commercial operations when the crash occurred, delaying indefinitely the start of passenger service.

George Whitesides, the head of the company dedicated to Branson’s vision of bringing everyday passengers into space, told the Financial Times the new fuel system used in Virgin’s SpaceShipTwo on Friday had been rigorously tested.

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.

On Saturday Branson vowed to find out what caused his space tourism company’s passenger spaceship to crash, but expressed a desire to press on with the dream of commercial space flight.

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.

“We owe it to our pilots to find out exactly what went wrong,” Branson said in Mojave on Saturday. “If we can overcome it, we will make absolutely certain that the dream lives on.”

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 U.S. offshoot of the London-based Virgin Group founded by Branson, whose empire ranges from airlines to music stores and mobiles phones.

(Editing by Gareth Jones)