(Bloomberg) — Sony Pictures Entertainment security advisersbelieve the cyberattack on the company's computer systems was thework of an organized group, Chief Executive Officer Michael Lyntonsaid in an e-mail to his staff.

Lynton said the investigation into the attack, which hasparalyzed the company's computers for almost two weeks, iscontinuing. Responding to staff concerns about security, he passedon a message he received from Kevin Mandia, chief operating officerof FireEye Inc., the cybersecurity firm assisting Culver City,California-based Sony Pictures in its investigation.

Mandia described the attack on Sony Corp.'s film and televisionstudio as unprecedented and said the malware used was undetectableby industry-standard antivirus software.

“This was an unparalleled and well planned crime, carried out byan organized group,” Mandia said in the e-mail, adding that neitherSony nor other companies “could have been fully prepared.”

Over the past week, a group calling itself “Guardians of Peace”paralyzed the U.S. unit's computers and disseminated thousands ofconfidential documents. Some contained thousands of Social Securitynumbers of employees and celebrities and sucked in other firms suchas the accounting firm Deloitte & Touche, which also as aresult had pay information leaked.

Threatening E-Mail

Studio employees recently received an e-mail from the purportedhackers, threatening their families and the studio. The e-mail,saying it's from the head of the Guardians of Peace, calls on Sonyemployees to sign their names at an e-mail address. It says theirfamilies will be in danger if they don't comply.

The studio said it's aware of the threatening e-mail and isworking with law enforcement. The Federal Bureau of Investigationsaid it's aware of the situation.

A Sony internal investigation has linked the hacking to a groupassociated with North Korea known as DarkSeoul, according to aperson familiar with the matter. DarkSeoul wiped out computers ofSouth Korean banks and broadcasters in March 2013.

North Korean supporters may have carried out the attack, thatcountry's news agency said today, citing an unidentified spokesmanat the National Defense Commission. While North Korea doesn't knowwhy Sony Pictures was targeted, it does know the company isproducing a film that defames its leadership, state- run KoreaCentral News Agency cited the spokesman as saying.

Sony Pictures is due to release “The Interview” on Dec. 25, anR-rated Seth Rogen comedy about a plot to kill North Korean leaderKim Jong Un.

'Mercilessly Destroy'

In June, North Korea promised to “mercilessly destroy” anyoneassociated with the film. The country's government described theproject as an “act of a war,” according to the BBC. An unnamedNorth Korean diplomat this week denied the government had any rolein the hacking, Voice of America reported.

The e-mail to employees written in broken English made thehackers' objective clear.

“Removing Sony Pictures on earth is a very tiny work for ourgroup which is a worldwide organization,” according to the e-mail,reported yesterday by industry magazine Variety.

In 2011, Tokyo-based Sony experienced a cyberattack that tookdown its PlayStation Network. In the breach, criminals stolepersonal information for 77 million customers and cost the company$171 million.

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