LOS ANGELES (AFP) - The surviving pilot of the Virgin Galactic spaceship that crashed last month unbuckled himself and was thrown free from the disintegrating craft, investigators said Wednesday.

Peter Siebold told them he did not know that his co-pilot had prematurely unlocked a key system on the spaceship, which broke up over California's Mojave Desert on Oct 31.

The co-pilot, Michael Alsbury, died in the accident shortly after mistakenly unlocking a so-called "feathering" system designed to slow the aircraft down during re-entry into the Earth's atmosphere.

The SpaceShipTwo broke up a few seconds after being dropped from its mothership, the WhiteKnightTwo, which had carried it up to an altitude of about 13,700m.

"According to the pilot, he was unaware that the feather system had been unlocked early by the co-pilot," the National Transportation Safety Board (NTSB) said in an update on its investigation.

The NTSB - which interviewed the injured pilot last Friday - added that his description of the vehicle motion was consistent with other data sources in the investigation.

"He stated that he was extracted from the vehicle as a result of the break-up sequence and unbuckled from his seat at some point before the parachute deployed automatically."

There was a two-stage system to deploy the feathering system: first a lock-unlock lever has to be released, and then the system itself deployed. The first step should not occur until the craft was above Mach 1.4, but investigators found it was unlocked too early, at a speed of above approximately Mach 1.0.

The accident was a serious blow to Virgin chief Richard Branson's dream of taking wealthy passengers up to the edge of space as tourists, and is likely to delay the program significantly.

It was the second disaster to rock the private sector space industry in less than a week, after an Antares rocket carrying supplies to the International Space Station exploded after takeoff in Virginia a few days previously.