MOJAVE, Calif. — Virgin Galactic’s experimental spaceship broke apart in flight over California’s Mojave Desert after a device to slow the craft’s descent prematurely deployed, federal investigators said Sunday.

National Transportation Safety Board Acting Chairman Christopher Hart said that while no cause for Friday’s crash of SpaceShipTwo has been determined, investigators found the “feathering” system — which lifts and rotates the tail to create drag — was activated before the craft reached the appropriate speed.

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.”

Hart said the investigation is months from being completed and pilot error and mechanical failure are among many things being looked at.