U.S. Air Force investigators said the reason for the malfunction of Boeing's Inertial Upper Stage (IUS) was that two stages of the IUS did not separate. The second stage, with the first one still attached, fired and started tumbling.

The Accident Investigation Board determined that the mission failed because thermal wrap and tape applied to the harness and connector prevented the proper disconnection of the plug. Technicians applied the thermal wrap to connectors according to the detailed operation procedures, or DOP, which provides detailed assembly instructions. Simply put: the thermal wrap probably shouldn't have been there anyway.

Aviation Week and Space Technology reported that a "remove before flight" tape was reportedly left on a connector by a technician, which caused the two stages of the Inertial Upper Stage rocket booster not to separate properly.

The partial separation of the two stages resulted in a series of cascading events. The IUS continued its stage 2 burn and spacecraft separation sequence, but the satellite was left in an inclined highly elliptical orbit. Because the satellite was not designed to compensate for an IUS failure, it failed to achieve the desired geosynchronous orbit. This resulted in the functional loss of the US$250 million payload.

• Boeing mishaps report
  

Launch failures chronology