WASHINGTON - Hackers stole the computer credentials of a system administrator to get access to Sony's computer system, allowing them broad access, said a report quoting US officials.
Officials briefed on the investigation told CNN that the hackers' ability to gain access to the passwords of a top-level information technology employee allowed them to have "keys to the entire building".
This is one reason why US investigators do not believe the attack on Sony was aided by someone on the inside, CNN reported, citing the officials.
The revelation is part of what is behind the government's conclusion that hackers operating on behalf of North Korea were responsible, according to the CNN report.
The TV station said the US government is expected to publicly blame the reclusive regime as early as Friday.
The cyber attack that forced Sony Pictures to scrap a comedy about North Korea is a "serious national security matter," the White House warned on Thursday, threatening an "appropriate response."
But White House spokesman Josh Earnest declined to confirm reports that North Korea had attacked the movie giant, which pulled the film after the Guardians of Peace (GOP) hacker group invoked Sept 11 in threatening attacks on cinemas, AFP reported.
But in a sign that US intelligence believed the attack came from an enemy of the United States, he said: "The president considers this to be a serious national security matter."
Earnest said the attack, which saw hackers gain access to a trove of internal Sony documents, was carried out by a "sophisticated actor."
"There has been destructive activity with malicious intent, and the administration believes that that activity merits an appropriate response from the United States," AFP quoted Earnest as saying.
With the Hollywood scandal threatening to escalate into geopolitical crisis, Sony defended its decision to cancel the release of "The Interview," a movie about a fictional CIA plot to kill North Korea President Kim Jong Un, according to AFP.
"This is bigger than us," the studio insisted, as footage that leaked onto the Internet showed that the film's climax was to have been a graphic close-up of the North Korean leader's fiery death.
- US options unclear -
It's unclear what measures the United States could take if it determines Pyongyang was behind the cyber attack against Sony. Experts argue North Korea could be testing the US and probing where it is vulnerable in cyberspace and as such, a strong response is needed, AFP reported.
"A weak response will only embolden North Korea and lead to more serious attacks, even if it is not proven to be the culprit," wrote Bruce Bennett, senior defence analyst with the Rand Corporation.
North Korea has denied involvement in the brazen Nov 24 cyber attack, which experts say could have been carried out by disgruntled Sony workers or by supporters of a foreign power.
Sony cancelled the film's Dec 25 release after major US theater chains said they would not screen "The Interview".
Free speech advocates and foreign policy hawks slammed the decision as cowardice in the face of a hidden enemy, AFP reported. Former presidential nominee Senator John McCain lashed Sony for backing down.
"By effectively yielding to aggressive acts of cyber-terrorism by North Korea, that decision sets a troubling precedent that will only empower and embolden bad actors to use cyber as an offensive weapon even more aggressively in the future," said McCain.
A Sony source said the Hollywood studio had no choice.
"This was a terrorist act, and you don't take that lightly," the source said, adding that "this is much bigger than us ... it's a whole new world, now warfare is on the cyber level."