New reports reveal what the FBI was hiding about the Sony hack

The trail that led American officials to blame North Korea for the destructive cyberattack on Sony Pictures Entertainment in November winds back to 2010, when the National Security Agency scrambled to break into the computer systems of a country considered one of the most impenetrable targets on earth. Spurred by growing concern about North Korea’s maturing capabilities, the American spy agency 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 former United States and foreign officials, computer experts later briefed on the operations and a newly disclosed N.S.A. document.