“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,” SpaceX said. “All plausible causes are being tracked in an extensive fault tree and carefully investigated.”
http://spaceflightnow.com/2016/09/23/falcon-9-rocket-explosion-traced-to-upper-stage-helium-system/











