Sony Pictures Entertainment has hired FireEye’s Mandiant forensics unit to clean up a cyber attack that knocked out the studio's computer network nearly a week ago, and resulted in three movies ending up online.

The FBI is also investigating the incident. Sony went down last Monday after displaying a red skull and the phrase "Hacked By #GOP," which reportedly stands for Guardians of Peace. Emails to Sony have been bouncing back with messages asking senders to call employees because the system was "experiencing a disruption."

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 holiday attack on Target. Sony is 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 Dec. 25 in the United States and Canada.

The movie is a comedy about a CIA attempt to assassinate North Korean leader Kim Jong Un, who is such a funny guy. 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.