HBO employees were working to uncover the source of four leaked episodes of its insanely popular and oft-pirated show "Game of Thrones" on torrent sharing sites a day before its fifth season premiere.

"Sadly, it seems the leaked four episodes ... of Game of Thrones originated within a group approved by HBO to receive them. We're actively assessing how this breach occurred," HBO said in a statement.

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The number of illegal downloads was expected to have exceeded a million. The premium channel recently launched a broadband service, "HBO Now," and decided to premiere the fifth season simultaneously in 170 countries to deter piracy.

Journalists who had received "screener copies" were the most likely culprits, writers speculated in many publications. But hackers might also be responsible for the leaks, cybersecurity expert Joseph Steinberg noted in "Forbes."