MOJAVE, Calif.—An improper co-pilot command preceded Friday’s in-flight breakup of Virgin Galactic LLC’s rocket, according to investigators, when movable tail surfaces deployed prematurely. (Latest:Virgin Galactic will proceed with second SpaceShipTwo despite crash).

Two seconds after the surfaces moved—with SpaceShip Two traveling faster than the speed of sound—“we saw disintegration” of the 60-foot-long experimental craft, according to Christopher Hart, acting chairman of the National Transportation Safety Board.

...