The Federal Bureau of Investigation has sent a warning to companies in the United States regarding hackers that have utilized malicious software for launching devastating attacks within the country.

The FBI issued the warning after the crippling hacking attack that Sony Pictures Entertainment suffered last week.

The attack left most of the company's employees logged out of their own computers. An investigation, meanwhile, has ensued.

The group of hackers responsible for the attack, who called themselves #GOP or Guardians of Peace, held the internal data that it obtained from the company's systems up for ransom and threatened to release the data unless their demands were met.

The confidential flash warning, which was five pages long, was released to companies by the FBI late Dec. 1 to be able to provide the businesses with technical details on the malware that was used for an attack. However, the document did not specify the victim of the malware.

Two experts in the field of cyber security, upon a review of the document, said that the victim being referred to was definitely Sony Pictures Entertainment.

"This correlates with information that many of us in the security industry have been tracking," said one of the experts who studied the document, adding that the information looks identical to that obtained from the attack on Sony Pictures Entertainment.

FBI spokesman Joshua Campbell declined to confirm that the malware detailed in the document was the one used against Sony Corp's California-based unit. However, he did confirm that the FBI was the source of the confidential flash warning.

According to Campbell, the agency regularly advises businesses regarding several possible cyber security threats that the FBI comes across. The data is provided by the FBI to companies to assist system administrators in safeguarding their respective businesses against the criminal actions of hackers.

The confidential flash warning detailed the recent malware that is able to override the data stored in the hard drives of computer systems, making them completely inoperable and shutting down entire networks.

"This malware has the capability to overwrite a victim host's master boot record and all data files," according to the report, adding that once the data has been overwritten, it is very costly and difficult to recover the lost data, if at all possible.

Similar forms of malware have been used by hackers in decapitating attacks against businesses in the Middle East and South Korea. Security experts said that if this was the same kind of malware that was used on Sony, the attack would have been the first large-scale one of its kind that has been carried out against a business operating in the U.S.