Hackers have stolen what HBO on Monday called “proprietary information” from the company’s computers. And though it has not disclosed what was taken, some news reports say the stolen goods may include new episodes of some series and also material related to Game of Thrones.

HBO chief executive Richard Plepler reported the breach in an email to employees Monday morning saying that “there has been a cyber incident directed at the company which has resulted in some stolen proprietary information, including some of our programming. Any intrusion of this nature is obviously disruptive, unsettling, and disturbing for all of us.”

Several news outlets, including Entertainment Weekly and Reuters, quoted an email from unidentified hackers claiming that they had taken 1.5 terabytes of information, including unspecified material from Game of Thrones.

One episode each of Ballers and Room 104 had been posted online but no episodes of Game of Thrones had yet appeared, according to Entertainment Weekly, which reported that the hackers may have taken a script or story treatment describing next week's episode of the show. HBO has gone to extraordinary pains to protect the coming plot twists from Game of Thrones from leaking to the public.

The statement from HBO did not detail when the incident happened, and the company declined to comment on reports detailing the material stolen.

In its statement, the company said, “HBO recently experienced a cyber incident, which resulted in the compromise of proprietary information. 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.”

“Game of Thrones” is an exceptionally valuable property for HBO, and HBO has gone to unusual lengths to prevent episodes of the series from leaking online in advance of their air date.

In its seventh season, the series has become the highest-rated cable program across all platforms, a particularly notable figure for a series that airs on a premium cable network rather than a basic cable one. 12.7 million tuned in to HBO itself for the seventh-season premiere, and another 6 million watched on streaming platforms like HBO Go, which is available to the network’s subscribers. HBO covers a substantial part of “Game of Thrones’” hefty production costs by selling licenses to air the series overseas and monetizes the show with a huge range of official merchandise. And most importantly at a moment of transition for the cable model, “Game of Thrones” is so popular that even people who don’t have cable will subscribe to HBO’s stand-alone service, HBO Now, in order to see it.

Given those figures and revenue streams, HBO has a strong interest in making sure that viewers can’t get their hands on “Game of Thrones” without an HBO subscription. After the first four episodes of the show’s fifth season leaked, the network stopped sending advance screeners even to critics. For the show’s seventh season, the only chance at an early look at the first episode was a single screening in Los Angeles.

Hackers have targeted Hollywood studios before. A group called The Dark Overlord demanded ransom from Netflix for its hack in April of the upcoming season of Orange is the New Black.

But the most devastating hack of a studio was 2014's intrusion into the computer systems of Sony Pictures, which led to the public release of embarrassing e-mails, employee payment records and other information. U.S. intelligence officials ultimately blamed North Korea, which was aggrieved over an upcoming Sony film The Interview, a comedy built about a fictional assassination plot against leader King Jong Un.