WASHINGTON - The detective work blaming North Korea for the Sony hacker break-in appears to be largely circumstantial, the Associated Press has learned. The dramatic conclusion of a Korean role is based on subtle clues in the hacking tools left behind and the involvement of at least one computer in Bolivia previously traced to other attacks blamed on the North Koreans.

Experts cautioned that hackers notoriously employ disinformation to throw investigators off their tracks, using borrowed tools, tampering with logs, and inserting false references to language or nationality.

The hackers are believed to have been conducting surveillance on the network at Sony Pictures Entertainment Inc. since at least the spring, based on computer forensic evidence and traffic analysis, a person with knowledge of the investigation told the AP.

The evidence has been considered conclusive enough that a U.S. official told the AP that federal investigators have connected the Sony hacking to North Korea.

In public, White House spokesman Josh Earnest on Thursday declined to blame North Korea, saying he didn't want to get ahead of investigations by the Justice Department and the FBI. Earnest said evidence shows the hacking was carried out by a "sophisticated actor" with "malicious intent."

An earlier formal statement by the White House National Security Council also did not name North Korea but noted that "criminals and foreign countries regularly seek to gain access to government and private sector networks" and said, "We are considering a range of options in weighing a potential response." The U.S. official who cited North Korea spoke on condition of anonymity because that official was not authorized to openly discuss an ongoing criminal case.

U.S. options against North Korea are limited. The United States already has a trade embargo in place, and there is no appetite for military action. Even if investigators could identify and prosecute the individual hackers believed responsible, there's no guarantee that any who are overseas would ever see a U.S. courtroom. Hacking back at North Korean targets by U.S. government experts could encourage further attacks against American targets.

Sony abruptly canceled the Dec. 25 release of its comedy, The Interview, an action the hackers had demanded partly because it included a scene depicting the assassination of North Korea's leader. Sony cited the hackers' threats of violence at movie theaters that planned to show the movie, although the Homeland Security Department said there was no credible intelligence of active plots. The hackers had been releasing onto the Internet huge amounts of highly sensitive - and sometimes embarrassing - confidential files they stole from inside Sony's computer network.

North Korea has publicly denied it was involved, though it has described the hack as a "righteous deed."

The episode is sure to cost Sony many millions of dollars. In addition to lost box-office revenue from the movie, the studio faces lawsuits by former employees angry over leaked Social Security numbers and other personal information.

Sony's decision to pull the film has raised concerns that capitulating to criminals will encourage more hacking.

"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 Sen. John McCain (R., Ariz).