The National Security Agency has been penetrating North Korea's computer networks since 2010, which helped prompt the U.S. to accuse Kim Jong-un's government of ordering the recent attack on Sony, the New York Times reported Sunday. The NSA used malware to tap into the Chinese networks that connect North Korea to the outside world and accessed the hackers' computers with the help of South Korea and other American allies, according to former U.S. and foreign officials and newly disclosed NSA documents. The NSA program was able to map the internal workings of North Korea's hackers, which totals about 6,000 people who are mostly commanded by the country's central intelligence service, the Reconnaissance General Bureau and its Bureau 121, the clandestine hacking unit. The latest revelation raises speculation over why the American spy agency wasn't able to alert Sony when North Korea began "spear phishing," or using malicious code concealed by email to penetrate computer systems, but the attacks did not look unusual at the time, the Times reports. Investigators have recently concluded that hackers worked from mid-September to mid-November to map out Sony's systems in a plot to bring down the network.