HBO has suffered a cyber attack which saw a Game Of Thrones script stolen and leaked online.
The network said 1.5 terabytes of data were stolen, including episodes of popular TV shows Ballers and the Duplass brothers' new comedy Room 104.
Hackers breached HBO's systems on Monday and threatened to return for more.
"HBO recently experienced a cyber incident, which resulted in the compromise of proprietary information," the company said in a statement.
"We immediately began investigating the incident and are working with law enforcement and outside cybersecurity firms. Data protection is a top priority at HBO, and we take seriously our responsibility to protect the data we hold."
In an anonymous email published by website Entertainment Weekly, a group claiming to be responsible for the attack wrote: "Hi to all mankind. The greatest leak of cyber space era is happening."
The network is the latest victim in a line of attacks targeting Hollywood studios, which has included Disney and Netflix.
In April, a hacker penetrated Netflix to release episodes from season 5 of Orange Is the New Black and, in May, hackers claimed to have stolen Pirates of the Caribbean: Dead Men Tell No Tales.
"Any intrusion of this nature is obviously disruptive, unsettling, and disturbing for all of us," said HBO CEO Richard Pepler.
"The problem before us is unfortunately all too familiar in the world we now find ourselves a part of. 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."
No episodes of Game Of Thrones have yet been released in video format and it is unclear which episode scripts have been leaked.