Why the U.S. was sure North Korea hacked Sony

WASHINGTON -- Long before blaming North Korea for the attack on Sony Pictures and its film "The Interview," officials say U.S. intelligence was monitoring North Korea's attempts to hack into computer systems of American companies.

Part of that effort involved a secret 2010 infiltration carried out by the U.S. National Security Agency, according to the New York Times. The NSA reportedly hacked into North Korean computer networks and inserted malware, allowing analysts to track the North's cyber operations.

Over time the U.S. was able to identify specific addresses used by hackers working for the North Korean government. Now those same computer IDs have been linked to threats sent to Sony, according to FBI Director James Comey.

"We could see that the IP addresses that were being used to post and to send the e-mails were coming from IPs that were exclusively used by the North Koreans," said Comey.

Comey revealed the evidence this month to a New York cyber conference to publicly bolster the U.S. case against North Korea. The regime has denied the attack and independent cyber analysts have questioned the FBI's conclusions. But Comey is not backing down.

"I have very high confidence about this attribution, as does the entire intelligence community," said Comey.

It's not clear whether U.S. intelligence was able to watch the attack against Sony play out in real time. But, the hackers were able to destroy computer systems, steal sensitive documents, and download several unreleased movies.