The National Transportation Safety Board (NTSB) says resulting aerodynamic forces caused the brakes to be actually deployed, tearing apart the craft.
The NTSB has been probing what caused the craft to break up over the Mojave Desert in a test flight 10 months ago.
The accident killed co-pilot Michael Alsbury and badly injured the pilot.
The Virgin Galactic's SpaceShipTwo space tourism craft was flying a manned test last October when it experienced what the company described at the time as "a serious anomaly".
It had been undergoing a powered test flight over the desert north of Los Angeles.
Virgin Group founder Sir Richard Branson said after the disaster that he was "shocked and saddened" by the "tragic loss".
NTSB chairman Christopher Hart said on Tuesday that he hoped the investigation would prevent a similar accident recurring, adding that the safety board had learned "with a high degree of certainty the events that resulted in the break-up".
"Many of the safety issues that we will hear about today arose not from the novelty of a space launch test flight, but from human factors that were already known elsewhere in transportation," he added.
Both pilots were employed by Scaled Composites, the company that designed the craft.
Investigators say a Virgin Galactic spaceship crash was caused by structural failure after the co-pilot unlocked a braking system early.