Citing anonymous sources in and close to the US government, the New York Times reports that the fingering of North Korea as responsible for the attack on the network of Sony Pictures Entertainment was through evidence gathered by National Security Agency surveillance. This includes software taps into networks associated with North Korea's network warfare and espionage unit, Bureau 121, among others. The actual evidence, however, will likely never see the light of day because of the highly classified nature of how it was obtained.
David Sanger and Martin Fackler of the Times report that the NSA started to ramp up efforts to penetrate North Korea's networks in 2010 to monitor the growth of Bureau 121 and the rest of the country's "computer network exploitation" capabilities:
A classified security agency program expanded into an ambitious effort, officials said, to place malware that could track the internal workings of many of the computers and networks used by the North’s hackers, a force that South Korea’s military recently said numbers roughly 6,000 people. Most are commanded by the country’s main intelligence service, called the Reconnaissance General Bureau, and Bureau 121, its secretive hacking unit, with a large outpost in China.
The evidence gathered by the “early warning radar” of software painstakingly hidden to monitor North Korea’s activities proved critical in persuading President Obama to accuse the government of Kim Jong-un of ordering the Sony attack, according to the officials and experts, who spoke on the condition of anonymity about the classified NSA operation.
The NSA's Tailored Access Office, according to the report, "drilled into the Chinese networks that connect North Korea to the outside world, picked through connections in Malaysia favored by North Korean hackers, and penetrated directly into the North with the help of South Korea and other American allies." According to NSA documents released by Der Spiegel, some of South Korea's initial assistance was not voluntary—the NSA secretly exploited South Korea's existing hacks of North Korea to gain intelligence information. But despite the level of access they gained, according to an unnamed investigator into the Sony Pictures attack, the NSA and other US agencies "couldn't really understand the severity" of the attack that would be launched against Sony when they began on November 24.