Reports say the amount of data stolen is believed to be about 1.5 terabytes.
Many reporters received an anonymous email on Sunday announcing the hack, according to EW.
ET has also obtained a letter from HBO chairman and CEO Richard Plepler to his employees, in which he calls the breach "disruptive, disturbing, and unsettling".
In a statement released Monday, network officials confirmed that "proprietary information" was compromised in a cyberattack.
Williams forced to wait for appeal verdict
The All Blacks centre was handed the suspension for his second Test send-off against the British & Irish Lions earlier this month. After a two-hour hearing on Tuesday night, an independent committee said it had reserved its decision.
Entertainment Weekly reported that several unaired episodes of HBO shows were put online, as well as what appeared to be at least one script or treatment from the popular "Game of Thrones" series. The leak happened after HBO mailed DVD copies of the episodes to members of the media for review, according to Forbes. "Data protection is a top priority at HBO, and we take seriously our responsibility to protect the data we hold", HBO said in a statement. In 2015, the first four episodes of the fifth season were leaked shortly after the season premiere. "I can assure you that senior leadership and our extraordinary technology team, along with outside experts, are working round the clock to protect our collective interests".
"The problem before us is unfortunately all too familiar in the world we now find ourselves a part of", he wrote.
Previously, Benioff and Weiss responded to the criticism by asking viewers to reserve their judgment, because "it's an ugly and a painful history, but we all think this is a reason to talk about it, not a reason to run from it".
"As has been the case with any challenge we have ever faced, I have absolutely no doubt that we will navigate our way through this successfully", the letter concluded.
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