Virgin Galactic Spaceship crash
Investigators from the National Transportation Safety Board (NTSB) inspect wreckage at one of the debris fields of the crash of Virgin Galactic's SpaceShipTwo near Cantil, California David McNew/Reuters

Investigators have said the probe over what caused the Virgin Galactic's SpaceShip Two to crash in California's Mojave Desert is likely to take a year.

The US National Transportation Safety Board [NTSB] which is leading the investigation would scour the crash site for the next week before interviewing people involved in the test flight.

Christopher Hart, the acting chairman of the board, said Virgin Galactic would be allowed to launch further test flights alongside the ongoing investigation.

"The on-scene part [of the probe] I would estimate probably four to seven days, and then we go off-scene and continue the factual collection," he told reporters.

"The total time... will be probably about 12 months or so, but again, as I say, this does not stop the operator from operating."

"Because it was a test flight it was heavily documented in ways that we don't usually see with normal accidents."

The spaceship disintegrated mid-air at about 45,000 feet just minutes after it was launched from the aircraft which was carrying it.

"When the wreckage is dispersed like that, it indicates the likelihood of inflight breakup," Hart said, adding that the debris of the ship is scattered as far as five miles across the desert.

The crash killed pilot Michael Tyner Alsbury, 39, and severely injured the co-pilot 43-year-old Peter Siebold. The mishap has dealt a major blow to Virgin Group founder Sir Richard Branson's dream space venture and space-tourism as a whole.

The British billionaire, who is at the launch-site in Mojave Air and Space Port, said: "We are determined to find out what went wrong."

"Yesterday, we fell short. We'll now comprehensively assess the results of the crash and are determined to learn from this and move forward."

Referring the questions pertaining to the cause of the incident to the investigative agencies, Branson said: "To be honest, I find it slightly irresponsible that people who know nothing about what they're saying can be saying things before the NTSB makes their comments."