Update (4:25 P.M.): HBO C.E.O. Richard Plepler told staff in a note Tuesday that the company does not believe its email system has been compromised, “but the forensic review is ongoing.” Per The Hollywood Reporter, the premium cabler is also working to block the leaked content from reaching viewers—including by sending a Digital Millennium Copyright Act take-down notice to Google, which is removing links to stolen material. HBO has used the DMCA to prevent the spread of copyrighted material for years, T.H.R. notes, “but the effort has not been nearly as aggressive as the network's post-hack offensive.”

The original post continues below.

It seems the recent HBO hack was just as bad as the hackers threatened it would be, according to [The Hollywood Reporter]http://www.hollywoodreporter.com/news/hbo-hack-insiders-fear-leaked-emails-as-probe-widens-1025827). When Netflix was hacked earlier this year, the cyber-criminals behind the attack demanded a ransom. But there was no such demand in the hack that struck HBO over the weekend, and the sheer amount of compromised data has led some to believe that video footage, internal documents, or e-mails could be leaked next. The premium-cable giant is working with the F.B.I. and cyber-security firm Mandiant to investigate the breach, in which hackers claimed to have stolen 1.5 terabytes’ worth of data. Per T.H.R., that would make this hack about seven times larger than the Sony attack in 2014, which buried the studio in leaked e-mails.

“A traditional business-grade D.S.L. link would take about two weeks at full blast to exfiltrate that much data,” Farsight Security C.E.O. Paul Vixie told T.H.R. “If not for video and sound, a corporation the size of HBO might fit [entirely] in a terabyte, including all the e-mail and spreadsheets ever written or stored.” Another expert added that the entire Library of Congress contains an estimate of 10 terabytes of print material—so it is almost certain that video and/or audio were stolen.

Incidentally, Mandiant, the cyber-security firm with whom HBO is reportedly working, led the Sony hack investigation as well.

HBO has not yet commented beyond its initial statement acknowledging the hack: “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 cyber-security firms. Data protection is a top priority at HBO, and we take seriously our responsibility to protect the data we hold.”

On Sunday, the hacker sent the following e-mail to multiple entertainment reporters, including several at Vanity Fair:

Hi to all mankind. The greatest leak of cyber space era is happening. What’s its name? Oh I forget to tell. Its HBO and Game of Thrones……!!!!!! You are lucky to be the first pioneers to witness and download the leak. Enjoy it & spread the words. Whoever spreads well, we will have an interview with him . . . HBO is falling.