Photo: The Canadian Press. All rights reserved.

This photo provided by Columbia Pictures - Sony shows, Randall Park, center, as Kim Jong Un in Columbia Pictures' "The Interview." (AP Photo/Columbia Pictures - Sony, Ed Araquel)

A U.S. official says North Korea has been linked to the unprecedented act of cyberwarfare against Sony Pictures that exposed tens of thousands of sensitive documents and escalated to threats of terrorist attacks that ultimately drove the studio to cancel all release plans for the film at the heart of the hack, "The Interview."

The attack is possibly the costliest for a U.S. company ever, said Avivah Litan, a cybersecurity analyst at research firm Gartner. "This attack went to the heart and core of Sony's business — and succeeded," she said. "We haven't seen any attack like this in the annals of U.S. breach history."

Federal investigators believe there is a connection between the Sony Pictures hack and the isolated communist nation, according to an official who spoke on condition of anonymity because the official was not authorized to openly discuss an ongoing criminal case. Earlier in the day, the besieged company cancelled the Christmas Day release of "The Interview," citing the threats of violence against movie theatres and decisions by the largest multiplex chains in North America to pull the film from its screens. Seemingly putting to rest any hope of a delayed theatrical release or a video-on-demand release, Sony Pictures later said it has "no further release plans for the film."

The cancellation was a startling blow to the Hollywood studio. The hackers, who call themselves Guardians of Peace, on Tuesday had threatened violence reminiscent of September 11th, 2001 against movie theatres showing the film. Sony cancelled a planned New York premiere and offered theatres the option of bowing out. One after the other, all the top U.S. movie chains announced they would postpone any showings of the comedy, which features a pair of journalists played by James Franco and Seth Rogen that are tasked by the CIA to assassinate North Korea leader Kim Jong Un. Sony said it then had little choice but to cancel the release.

"We are deeply saddened at this brazen effort to suppress the distribution of a movie, and in the process do damage to our company, our employees, and the American public," Sony Pictures said in a statement Wednesday. "We stand by our filmmakers and their right to free expression and are extremely disappointed by this outcome."

White House spokeswoman Bernadette Meehan said the U.S. government had no involvement in Sony's decision, adding that artists and entertainers have the right to produce and distribute whatever content they want in the U.S.

"We take very seriously any attempt to threaten or limit artists' freedom of speech or of expression," Meehan said.

Just how much the cyberattack will ultimately cost Sony is unclear. Sony faces trouble on several fronts after nearly four weeks since the hackers first crippled its computer systems and started dumping thousands of emails and private documents online.

In addition to vanishing box-office revenue from "The Interview," leaked documents could muck up production schedules, experts say. There will be the cost of defending the studio against multiple lawsuits by ex-employees angry over leaked Social Security numbers and other personal information. And then there are actors and talent who just might decide to work at another studio.

Beyond the financial blow, some say the attack and Sony's capitulation has raised troubling questions about self-censorship and whether other studios and U.S. companies are now open season for cyberterrorists.

"Artistic freedom is at risk," said Efraim Levy, a senior financial analyst at research firm S&P Capital IQ. "Are we not going to put out movies that offend some constituencies?"