The spectacular nighttime explosion of a Delta III rocket Wednesday was caused by a steering system failure, officials said Friday.
The inaugural flight of Boeing's new rocket ended 71 seconds after liftoff when the control system in three solid propellant boosters ran out of hydraulic fluid.
The rocket then deviated from its flight path, broke up and finally exploded.
Why the boosters prematurely used up all the hydraulic fluid remained unclear.
The mission was valued at $225 million.
The satellite carried by the rocket crashed in a fireball into the Atlantic Ocean 15 miles offshore.
Boeing said that, as a precaution, it will delay Tuesday's scheduled launch of a Delta II from Vandenberg Air Force Base, Calif., by a few days.
The rocket is to carry five Iridium satellites into orbit.