FBI sources: Sony Pictures cyberattack traced to North Korea

President Obama's administration continued grappling on Thursday with the question of whether to formally accuse North Korea of involvement in the cyberattack on Sony Pictures, with a White House spokesman stopping short of directly blaming the North Koreans, but FBI sources say the trail points pretty clearly to the rogue regime's involvement in some manner.

CBS News Homeland Security Corresponent Bob Orr reports that sources say the FBI has definitively traced the attack back to the regime of North Korean leader Kim Jong-Un. Evidence shows hackers directed by North Korea's cyber unit used aggressive "data-wiping" malware to steal Sony's corporate secrets and then erase the company's computer files.

According to Orr, investigators found similarities between the Sony hack and a broadly damaging 2013 cyber assault on South Korean banks and broadcasters. Sources say the digital fingerprints in both of those cases have been traced back to the North.

The attack on Sony, conducted by a group calling itself the Guardians of Peace, exposed a number of embarrassing emails between producers and top executives at the movie studio. The group said it was targeting Sony because of its film "The Interview," a comedy about a pair of American journalists sent to assassinate North Korean dictator Kim Jong Un.

At a White House press briefing earlier on Thursday, spokesman Josh Earnest stopped short of directly accusing North Korea of involvement in the cyberattack, but he said the investigation was "progressing," and the available evidence indicates the attack was "initiated by a sophisticated actor."

"They did carry out a destructive activity with malicious intent, and that is something that this administration takes very seriously," he said. "We view it as a legitimate national security matter."

The threat of additional attacks scuttled "The Interview's" planned Christmas Day release. Major movie theater chains like Regal and AMC announced Wednesday that they would not show the movie, prompting Sony to announce it would not move forward with the debut.

In a statement, the studio decried the hack as an "unprecedented criminal assault against our employees, our customers, and our business."

"Those who attacked us stole our intellectual property, private emails, and sensitive and proprietary material, and sought to destroy our spirit and our morale - all apparently to thwart the release of a movie they did not like," the statement read. "We stand by our filmmakers and their right to free expression and are extremely disappointed by this outcome."

The North Korean state-run news agency has described the making of "The Interview" as an "act of war" and promised a "merciless response." While the North Korean government has labeled the Sony hack a "righteous deed," it has not claimed responsibility for the intrusion.

The rogue regime has been blamed for a number of serious cyberattacks before, many of them directed at its neighbor, South Korea, CBS News correspondent Seth Doane reports. A congressional report last year claimed that the North Koreans have launched more than 6,000 cyberattacks since 2010. One attack in March of 2013 destabilized the computers and servers of South Korea's major TV networks, affecting more than 30,000 computers. The South Korean army believes there are roughly 3,000 hackers in North Korea.

Previous cyberattacks on American corporations have prompted an aggressive response from U.S. officials. In May, the Justice Department issued indictments for five Chinese military officers, accusing them of stealing trade secrets and sensitive proprietary information from U.S. solar and nuclear companies. Despite the charges, none of the accused hackers have yet been extradited or prosecuted.

In the wake of the cancellation of the film and reports of North Korean involvement in the hack, several key lawmakers promised an aggressive response.

Sen. John McCain, R-Arizona, said the links between North Korea and the Sony hack "are profoundly troubling."

""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," McCain said. He also reserved some blame for the administration's "continuing failure to satisfactorily address the use of cyber weapons by our nation's enemies."

In an interview with ABC News that aired Wednesday, President Obama says the attack on Sony shows "a lot more progress needs to be done" regarding U.S. cybersecurity efforts.

"That means also, by the way, that congress needs to take up cybersecurity legislation that's been languishing for several years now," he said, "because there's a bunch of stuff we can do to improve information sharing and make sure that these sites are hardened."

The president said there is no "credible evidence of any serious threats to theaters, or some sort of terrorist attack against theaters that are screening the particular movie at issue," but he added, "We'll be vigilant. If we see something that we think is serious and credible then we'll alert the public."