A U.S. official says North Korea perpetrated an 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 Interview," the film at the heart of the attack.

Federal investigators have now connected the Sony Pictures hack to 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 said it was canceling the Christmas Day release of "The Interview."

The attack is unprecedented and possibly the costliest for a U.S. company ever, says Avivah Litan, a cybersecurity analyst at research firm Gartner.

In a statement Wednesday, Sony said it was cancelling "The Interview" release "in light of the decision by the majority of our exhibitors not to show the film." The studio, which has been shaken by hacker leaks over the past several weeks, said it respected and shared in the exhibitors' concerns.

"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," read the statement. "We stand by our filmmakers and their right to free expression and are extremely disappointed by this outcome."

Earlier Wednesday, Regal Cinemas, AMC Entertainment and Cinemark Theatres the three top theater chains in North America announced that they were postponing any showings of "The Interview," a comedy about a TV host (James Franco) and producer (Seth Rogen) tasked by the CIA to assassinate North Korea leader Kim Jung-un (played by Randall Park).

Regal said in a statement that it was delaying "The Interview" ''due to wavering support of the film ... by Sony Pictures, as well as the ambiguous nature of any real or perceived security threats."

Sony had offered theaters the option of bowing out, and when so many of them did (other chains included ArcLight Cinemas, Cineplex Entertainment and Carmike Cinemas), it left Sony little choice for the release of "The Interview."

The seriousness of the threat made Tuesday in messages posted online by the hacking group that calls itself Guardians of the Peace, is unclear. The warning did prompt law enforcement in New York and Los Angeles to address measures to ramp up security.

Sony did not say what its plans for "The Interview" now are, or whether the film's release could potentially happen at a later date. Conjecture has centered on the possibility of an unprecedented on-demand release that would distribute the film without risk to theater operators. No wide-release studio film has ever been first released on VOD, out of protection of the theater business.

A representative for the film's directors, Seth Rogen and Evan Goldberg, didn't immediately respond to messages Wednesday.