NEW YORK

- 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 canceled the Christmas Day release of "The Interview," citing the threats of violence against movie theaters 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 Sept. 11, 2001, against movie theaters showing the film. Sony canceled a planned New York premiere and offered theaters 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 who 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 yesterday. "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.