HBO's chief said on Wednesday that the company is conducting a forensic review of its computer systems to better understand the scope of a cyber attack that surfaced over the weekend.

HBO, which acknowledged Monday that hackers had broken into its systems and stolen "proprietary information", now says the attackers likely haven't breached the network's entire email system.

A coordinated cyberattack on July 27 saw hackers walk away with one-and-a-half terabytes of HBO data, a publication reported, which included video footage, internal documents and email correspondence.

HBO parent company Time Warner is in the midst of an $85 billion merger with AT&T, which may be near closing.

"We are also in the process of engaging an outside firm to work with our employees to provide credit monitoring and we will be following up with those details", Plepler wrote in the email obtained by CNN Tech. The hackers proceeded to leak unreleased episodes of a handful of shows, and promised to leak more data in the coming days.

On Tuesday, a company called IP-Echelon filed a report with Google on behalf of HBO, noting that the named website "shares thousands of Home Box Office (HBO) internal company documents".

The company's investigation is still ongoing. Sony struggled in the aftermath of its huge hack in 2014, which leaked embarrassing employee emails as well as films, along with their personal financial data.

Sometimes when hackers steal data from companies, they seek a ransom in exchange for not publishing the data.

"Game of Thrones" spoilers may now be the least of HBO's worries this season.

Plepler warned employees that speculation in the media about the hack "may very well not be true".