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MOJAVE, California--A human-factors expert will join a team investigating the fatal test-flight crash of the Virgin Galactic passenger rocket plane to study why the co-pilot prematurely unlocked the ship's pivoting tail section, the National Transportation Safety Board chairman said on Monday.

The untimely engagement of the tail mechanism, designed to slow the vehicle's descent into the atmosphere from space, and the possibility that pilot error was to blame, were disclosed by the NTSB late on Sunday and have emerged as a main thrust of the inquiry into the accident.

The suborbital rocket vehicle dubbed SpaceShipTwo was undergoing its first powered test flight since January when it crashed on Friday shortly after separation from the special jet aircraft that carried it aloft for its high-altitude launch. Video footage from the cockpit shows co-pilot Michael Alsbury, 39, who died in the crash, releasing a lever to unlock the twin-tail section too soon after the space plane was jettisoned from the mother ship over Southern California.

This was followed two seconds later by premature movement of the tail, which pivots upward from the rear of the wings at a 90-degree angle and increases aerodynamic drag on the spacecraft.

Still, NTSB Chairman Christopher Hart told Reuters investigators have yet to determine whether releasing the tail mechanism too early caused or contributed to the crash of the space plane in California's Mojave Desert, about 95 miles north of Los Angeles.

"We know already from having the lever move from lock to unlock that we need to get a human-factors person in here because the question then is why did that happen when it happened," Hart said. "The human-factors person will be here today."

Investigators also are trying to determine how surviving pilot Pete Siebold, 43, managed to get out of the rocket plane and parachute to the ground from an altitude of roughly 50,000 feet, an altitude virtually devoid of oxygen. Hart said Siebold, who was hospitalized with a shoulder injury, was sitting in the right-hand seat, so it would have been difficult if not impossible for him to have escaped through a hatch in the lower left-hand side of the cockpit.

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.

Hart told a news conference on Sunday investigators had determined that the spacecraft's tail system was supposed to have been released for deployment as the craft was traveling about 1.4 times the speed of sound. Instead, the tail section began pivoting when the vehicle was flying at Mach 1, the speed of sound.

"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.