(Reuters) – A team investigating the fatal test-flight crash of a Virgin Galactic passenger spaceship found that its rotating tail, designed to ease reentry into the atmosphere, was activated prematurely, and said pilot error could not be ruled out.

The preliminary findings of the National Transportation Safety Board were disclosed late Sunday by its acting chairman, Christopher Hart, but he said it was too soon to know whether the untimely unlocking of the tail mechanism caused Friday’s accident.

The suborbital rocket plane dubbed SpaceShipTwo was undergoing its first powered test flight since January over California’s Mojave Desert when it crashed shortly after the craft separated from the special jet aircraft that had carried it aloft for its high-altitude launch.

One pilot was killed, and the other survived with serious injuries after ejecting from the spacecraft and parachuting to the ground.

SpaceShipTwo, developed by the fledgling space tourism company of billionaire entrepreneur Richard Branson, was designed to carry wealthy passengers on short rides into space, with Virgin Galactic planning to begin offering its first flights to paying customers next spring.

The crash came three days after the unmanned rocket of another private space company, Orbital Sciences Corp , exploded during liftoff from a commercial launch pad in Virginia on a mission to deliver cargo to the International Space Station under contract with NASA.

On Monday, Branson vowed to determine the cause of Friday’s crash and correct the problem. “We need to know exactly what happened to make absolutely certain it will never happen again,” he told CBS’s “This Morning” program.

NTSB’s Hart told a news conference on Sunday investigators had determined that the spacecraft’s ‘feathering’ tail system, designed to help slow its descent into the atmosphere from space, was supposed to have been released for deployment as the craft 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, the speed of sound.

Hart said the feathering tail system, which folds the vehicle in half aerodynamically to create more atmospheric drag, appeared from video footage to have been unlocked early by the co-pilot. About two seconds later, the space plane’s tail section began to rotate.

“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 pilot error was a possible factor, Hart said: “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.”

Branson early NTSB findings indicated to him that premature tail deployment could be a possibility.

“They’re indicating that that may well be the cause, but we need them to examine it further and let us know,” he told NBC’s “Today” show.

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