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.

Entertainment Weekly reported on Monday that hackers stole 1.5 terabytes of data and leaked online a script for an upcoming episode of the network’s popular series Game of Thrones, along with yet-to-be telecasted episodes of the series Ballers and Room 104. 

The hack comes at a sensitive time for HBO, as its parent company Time Warner Inc is waiting for regulatory approval to sell itself to AT&T Inc decided in an $85.4 billion deal announced in October.

Details of the hack have not been discussed.

Chairman Richard Plepler told employees in a Monday email seen by Reuters that the hackers had stolen “proprietary information,” as well as some programming.

Enclosed in his note on Wednesday, he said that some employees had expressed concern about the email system.

At this time, we do not believe that our email system as a whole has been compromised, but the forensic review is ongoing,” he said.

Plepler’s note read that HBO would hire an outside firm to provide credit monitoring for employees, but did not explain the reasons why.