SpaceX not to blame for secret Zuma satellite flop: Sources

Just what happened to the top-secret Zuma satellite launched by SpaceX this past January is still a mystery -- but the investigation into its failure has cleared SpaceX of any fault for the satellite failure.

Sources close to the investigation tell ABC News the fault lies with Northrup Grumman, the company responsible for building the satellite. Northrup Grumman has not responded to requests for comment.

SpaceX President and COO Gwynne Shotwell from the very beginning asserted that the company's Falcon 9 rocket performed as expected: “For clarity: after review of all data to date, Falcon 9 did everything correctly."

The launch was the first of the year for SpaceX but it was shrouded in such mystery because of the top-secret nature of its payload, code-named Zuma.

Little was known about Zuma, but it was clear after launch that the satellite did not reach orbit and is believed to have plunged back down to Earth, landing in the Indian Ocean.

Rocket launches are tough – the history of spaceflight has multiple incidents of explosions and SpaceX has had its fair share of failures.

One rocket bound for the space station disintegrated shortly after launch destroying supplies headed for the space station.

Another exploded on the launch pad during a static fire test. Landing rockets is even harder though SpaceX has racked up several successes.

But given the secrecy surrounding the Zuma launch most of the details of the failure will remain a closely guarded secret.