The U.S. National Security Agency has had a secret foothold for years in North Koreas networks and saw signs of the Sony Pictures Entertainment attack but only in retrospect grasped its reach and depth, The New York Times reported Sunday.

The spy agency has worked for at least four years to infiltrate networks inside North Korea and those in China and Malaysia favored by the countrys hackers, the newspaper reported, citing former U.S. and foreign officials and a newly disclosed NSA document published by Der Spiegel.

The revelation explains why the U.S. quickly blamed North Korea for the attacks despite widespread skepticism from the computer security community, which said only circumstantial evidence pointed to the countrys involvement.

The hackers were incredibly careful, and patient, the Times reported, citing a person who had been briefed on the investigation.

The Sony attack stole terabytes of sensitive documents, including a salary spreadsheet for 6,000 employees, internal emails, pre-release copies of films and vast amounts of personnel data. It also broke thousands of the organizations computers by using a destructive type of malicious software that wipes files.

A group calling itself the Guardians of Peace claimed responsibility for the attacks, releasing the data piecemeal on file-sharing sites and reaching out directly to journalists with links to the material.

It initially appeared the group wanted to blackmail Sony. Only later did the North Korean connection emerge in part due to Sony Pictures plan to release The Interview, a comedy centered on an absurd campaign by two Americans to assassinate North Korean leader Kim Jong Un.

After the U.S. blamed North Korea in mid-December, it was silent on what evidence led to the conclusion. On Jan. 2, President Barack Obama authorized sanctions against North Korea, adding to those in place for years against the secretive nation.

Its the second time the U.S. has directly blamed another country for cyberattacks. In the first legal action of its kind in May 2014, federal prosecutors charged five members of the Chinese Army with stealing trade secrets from U.S. organizations over eight years. China denied the accusations.

FBI Director James Comey offered more clues for the Sony attacks on Jan. 7, saying the hackers failed to to mask their IP addresses. That revealed some emails from the hackers to Sony employees came from Internet connections used by the North.

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Report: Inside North Korea's network, NSA saw signs of Sony attack