Sony Pictures Entertainment has reportedly hired FireEye's Mandiant forensics unit to investigate a massive cyber attack it experienced almost a week ago.

Mandiant is acyber security firm that became famous for its report against China in cyber espionage in 2013 involving the People's Liberation Army; the attack targeted 141 organizations based in the United States and other countries. The company was acquired by FireEye later on for more than $1 billion.

Three people familiar with the matter told Reuters that the FBI has also joined the investigation. Technicians are still fixing the hacked servers and email access is expected to be fully restored on Monday.

The Los Angeles Times reported almost a week ago that Sony's computer systems became useless after the cyber attack. Employees were unable to login to their email accounts for almost a day, and those who attempted to access were welcomed with a message "Hacked By #GOP" (Guardians of Peace). The group threatened Sony Pictures that it would release its secrets to the Web.

Following the attack, Variety reported that the hackers released to the Web the DVD screener versions of new movies titled "Fury," "Annie," "Mr. Turner," "Still Alice," and "To Write Love on her Arms." A source said that these copies were obtained during the hack last week. "The Fury" is not even in theaters yet but more than 1.2 million copies have already been downloaded.

Sony is investigating the possibility that North Korea is behind the attack, a source told Re/code. North Korea became a suspect because the attack coincided with the pending release of "The Interview," a comedy film made by Sony telling a story of the CIA attempting to assassinate North Korean leader Kim Jong-Un.

Representatives of FireEye and the FBI both refused to comment about the alleged investigation.