Sony Pictures Entertainment has hired FireEye’s Mandiant forensics unit to clean up a massive cyber attack that knocked out the studio’s computer network nearly a week ago.

The US Federal Bureau of Investigation is also investigating the incident.

The computer systems at the Sony unit went down last Monday after displaying a red skull and the phrase “Hacked By #GOP,” which reportedly stands for Guardians of Peace.

Technicians are making headway in repairing damage caused by the attack and expect to have the email systems back online Monday.

Sony executives have declined to comment on the scope of the attack, though emails have been bouncing back with messages asking senders to call employees because the system was “experiencing a disruption.”

Representatives with FireEye and the FBI declined comment.

Mandiant is an incident response firm that helps victims of breaches identify the extent of attacks, clean up networks and restore systems. The firm has handled some of the largest breaches uncovered to date, including the 2013 attack on Target.

Technology news website Re/code reported on Friday that Sony was investigating to determine whether hackers working on behalf of North Korea have launched the attack in retribution for the studio’s backing of the film “The Interview,” which is to be released on December 25 in the United States and Canada.

The movie is a comedy about a CIA attempt to assassinate North Korean leader Kim Jong Un.

The Pyongyang government denounced the film as “undisguised sponsoring of terrorism, as well as an act of war” in a letter to UN Secretary-General Ban Ki-moon.