"The forensic review is ongoing," HBO said in an emailed statement, noting that it was working with cyber security experts and law enforcement.
The Guardian and other publications reported on Tuesday that hackers had posted some stolen HBO files online, demanding a Bitcoin ransom to prevent additional releases.
Reuters was not able to view the leaked files or verify their authenticity.
Technology news site Mashable posted written demands that it said hackers claimed to have sent to HBO Chief Executive Officer Richard Plepler. The hackers said they had stolen a wide variety of confidential material including scripts to its programs, contracts, human resources documents and budget details.
A source familiar with the situation said the Federal Bureau of Investigation was probing the matter. An FBI spokesperson declined to comment.
It was the second time documents have been released on the Internet that were purportedly stolen from HBO since the hack surfaced early last week. Entertainment Weekly reported last week that hackers stole 1.5 terabytes of data and leaked online a script or treatment for an upcoming episode of the network's popular series Game of Thrones, along with yet-to-be-broadcast episodes of the series Ballers and Room 104.
HBO has provided few details about the hack. It said in its statement that it does not believe its "e-mail as a whole" was compromised, and added that it believed further leaks might emerge from the incident.
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