Sony Pictures Entertainment has hired fireeye's mandiant forensic department to investigate a massive cyber attack that caused the company's computer network to crash about a week ago, three sources familiar with the situation said on Sunday, Reuters reported. The FBI is also investigating the incident, according to a source not authorized to speak publicly about the handling of the attack.

The Los Angeles Times once reported that the computer system of Sony Pictures Entertainment company was attacked last Monday, and the red skull and the words "hacked by GOP" appeared on the computer screen. It is said that GOP is the abbreviation of guardians of peace.

Technicians are repairing the computer crash caused by the attack and are expected to restore the e-mail system on Monday, the person said. Sony executives declined to comment on the scope of the attack, but all emails sent were returned, along with emails asking senders to call each other because the system was "down". Representatives of both fireeye and the FBI declined to comment.

Mandiant is an event response company that helps victims determine the scope of attacks, clean up the network and recover the system. It has handled some of the largest network attacks, such as the attack on target during the 2013 holiday season. On Friday, technology news website re / code reported that Sony was investigating whether North Korea had launched the attack, because the company's film "the interview" (also known as the assassination of Kim Jong Un) will be released in the United States and Canada on December 25.

The movie is a comedy about the CIA's assassination of North Korean leader Kim Jong Un. In a letter to U.N. Security Council Secretary General Ban Ki Moon, the Pyongyang government condemned the film as "an explicit support for terrorism and an act of war.".