Virgin Galactic's SpaceShipTwo broke apart over California's Mojave Desert on Friday after a device to slow the experimental space tourism rocket's descent deployed too soon, according to investigators.

The accident killed the co-pilot, Michael Alsbury; the pilot, Peter Siebold, who piloted the mission, survived after parachuting out of the plane. On Sunday, Siebold was described as alert and talking with his family and doctors.

National Transportation Safety Board Acting Chairman Christopher Hart said that while no cause for Friday's crash of Virgin Galactic's SpaceShipTwo has been determined, investigators found the "feathering" system – which rotates the tail to create drag – was activated before the craft reached the appropriate speed.

Longtime flight engineer and test pilot Michael Alsbury was killed when the plane broke apart.

(AP)

The system requires a two-step process to deploy. The co-pilot unlocked the system but Hart said the second step occurred "without being commanded."

"What we know is that after it was unlocked, the feathers moved into the deploy position and two seconds later we saw disintegration," Hart said.

The finding moves away from initial speculation that an explosion brought down the craft, but still doesn't fully explain why the disaster occurred.

Hart said a review of footage from a camera mounted to the ceiling of the cockpit shows the co-pilot moving the feathering lever to the unlock position.

The feathering is a feature unique to the craft to help it slow as it re-enters the atmosphere. After being unlocked, a lever must be pulled to rotate the tail section toward a nearly vertical position to act as a rudder. After decelerating, the pilots reconfigure the tail section to its normal position so the craft can glide to Earth.

Hart said the feathers activated at Mach 1.0, the speed of sound or 760 mph. They shouldn't have deployed until the craft had at least reached a speed of Mach 1.4, or more than 1,000 mph.

The investigation is months from being completed and pilot error, mechanical failure, the design and whether there was pressure to continue testing are among many things being looked at, Hart said.

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Co-pilot Michael Alsbury was as capable behind the controls of experimental aircraft as he was tackling technical challenges in the offices where the vehicle was designed, colleagues said.

Alsbury, 39, was also known as a devoted husband and father of two young children.

Alsbury was "a respected and devoted colleague," according to a statement released over the weekend from Scaled Composites, the company developing the spaceship for Virgin Galactic.

"Without mincing words or really embellishing anything ... I consider Mike Alsbury the renaissance man," said Brian Binnie, another test pilot who worked at Scaled Composites for 14 years before leaving the company in February. "He could do it all. He was an engineer. He was a pilot. He worked well with others. He had a great sense of humor. I never heard him raise his voice or lose his cool."

At Scaled Composites, Alsbury participated in the flight testing of nine different manned aircraft and co-piloted SpaceShipTwo when it broke the sound barrier during its first powered flight last year. He was also sitting in the co-pilot's seat when the craft first dropped in 2010 from its carrier aircraft several miles above the Earth for an unpowered glide test.

Alsbury flew primarily as the craft's co-pilot, logging at least seven trips from 2010 to early 2014, according to test-flight logs.

Binnie said Alsbury was a "home-schooled, home-brewed" pilot who earned his way up through the ranks at the company, starting as an engineer. Alsbury had also put himself through commercial pilot school and was certified as a flight instructor.

"He was definitely the kind of guy who had a vision for himself, and he did what he thought would get him where needed to be to fly SpaceShipTwo," Binnie said. "Mike loved what he did. I think his career ended with him doing exactly that."

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