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The Virgin Galactic space plane that broke apart over the Mojave Desert on Friday shifted early into a high-drag configuration that is designed to slow it down, federal accident investigators have said.

The accident killed the co-pilot, Michael Alsbury; the pilot, Peter Siebold survived after parachuting out of the plane.

The craft, called SpaceShipTwo, is designed to rocket up, and when it reaches the top of its ascent, two tail booms rotate upward into a “feathered” position. That creates more drag and stability, allowing the plane to descend gently back into the atmosphere, much like a badminton shuttlecock.

At the news conference Sunday night at the Mojave Air and Space Port in California, Christopher A. Hart, the acting chairman of the National Transportation Safety Board, said the plane was not supposed to move into the feathered configuration unless the pilots take two actions: first, a lever is switched to unlock the tail booms, and then a handle is moved to feather the booms.

“About nine seconds after the engine ignited, the telemetry data told us the feather parameters changed from locked to unlocked,” Hart said.

In addition, a video camera in the cockpit showed Alsbury switching the lever to the unlocked position, Hart said. That occurred at a velocity of about Mach 1, which is the speed of sound at a given altitude. Under normal operations, that lever is not moved until later in the flight, when the space plane has reached a speed of Mach 1.4, Hart said. The plane’s altitude would also be higher, where the air is thinner.

Two seconds later, the booms rotated even though neither pilot moved the feathering handle.

“Shortly after the feathering occurred, the telemetry data terminated, and the video data terminated,” Hart said. “The engine burn was normal up until the extension of the feathers.”

Virgin Galactic, founded by the billionaire entrepreneur Richard Branson, is to take space tourists on short rides to an altitude of 62 miles, which is considered the boundary of outer space.

Technical issues have pushed back the start of the commercial flights. In September, Branson had said he hoped commercial flights would begin next spring.

In a statement Sunday, Virgin Galactic responded to criticism that the design of SpaceShipTwo was flawed and that the test flights were reckless.

“At Virgin Galactic, we are dedicated to opening the space frontier, while keeping safety as our ‘North Star’,” the company said. “This has guided every decision we have made over the past decade, and any suggestion to the contrary is categorically untrue.”