Three weeks after one of its rockets exploded while on a launch pad at Cape Canaveral, SpaceX said Friday that it is narrowing in on the possible cause, pinpointing a "large breach" in a helium system in the rocket's second-stage fuel tank.

In a statement, the company said the investigation is still "preliminary" and that "all plausible causes are being tracked in an extensive fault tree and carefully investigated."

The investigative team--led by SpaceX and comprised of the Federal Aviation Administration, NASA and the Air Force-- has looked through reams of data and evidence in trying to understand how the rocket suddenly ignited into a massive fireball while being fueled in preparation for an engine test, the company said.

On Twitter, Elon Musk, the company's founder, has said the failure was the most difficult and complicated the company has ever faced. And he asked the public to turn over any video or recordings of the event that might help investigators.

In the statement Friday, the company said, "the majority of debris from the incident has been recovered, photographed, labeled and cataloged, and is now in a hangar for inspection and use during the investigation...At this stage of the investigation, preliminary review of the data and debris suggests that a large breach in the cryogenic helium system of the second stage liquid oxygen tank took place."

It was not clear what caused the breach.

Last year, the company lost another Falcon 9 rocket when it blew up a couple minutes into flight. SpaceX said that explosion was also due to a problem with the upper stage. But on Friday it said it has ruled out any connection with that failure.