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 October 31.

The co-pilot, Michael Alsbury, died in the accident shortly after mistakenly unlocking a so-called "feathering" system designed to slow the aircraft 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 45,000 feet (13,700 meters).

"According to the pilot, he was unaware that the feather system had been unlocked early by the co-pilot," the National Transportation Safety Board 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.

But last week a company spokeswoman said Virgin Galactic could resume test flights with a new spaceship within six months. The NTSB probe is expected to last up to a year or more.

"It's possible that test flights for the next spaceship could begin within six months, before the investigation is expected to conclude," the Virgin spokeswoman told AFP.

New test flights would take place in a second SpaceShipTwo, which is 65 percent complete, the company said Tuesday.

Hundreds of VIPs, including Leonardo DiCaprio and Demi Moore, are said to have put down a $250,000 (200,000 euro) deposit on a space flight.

Virgin Galactic said there was a "small percentage requesting refunds... due to personal circumstances".

Branson, who rushed to the Mojave Desert within hours of the crash, has vowed to press on with his dream.

"While this has been a tragic setback, we are moving forward and will do so deliberately and with determination," his company said.

"We owe it to all of those who have risked and given so much to stay the course and deliver on the promise of creating the first commercial spaceline."

The crash was the second disaster to rock the private sector space industry in the same week.

An Antares rocket carrying supplies to the International Space Station exploded after takeoff in Virginia a few days previously.