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WASHINGTON — The trail that led US 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 US 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 US allies, according to former US and foreign officials, computer experts later briefed on the operations, and a newly disclosed NSA document.

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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, said the officials and experts, who spoke on the condition of anonymity about the classified NSA operation.

Obama’s decision to accuse North Korea of ordering the largest destructive attack against an American target — and to promise retaliation, which has begun in the form of new economic sanctions — was highly unusual: The United States had never explicitly charged another government with mounting a cyberattack on American targets.

Obama is cautious in drawing stark conclusions from intelligence, aides say. But in this case “he had no doubt,” one senior US military official said.

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“Attributing where attacks come from is incredibly difficult and slow,” said James A. Lewis, a cyberwarfare expert at the Center for Strategic and International Studies in Washington. “The speed and certainty with which the United States made its determinations about North Korea told you that something was different here — that they had some kind of inside view.”

The extensive American penetration of the North Korean system also raises questions about why the United States was not able to alert Sony as the attacks took shape last fall, even though the North had warned, as early as June, that the release of the movie “The Interview,” a crude comedy about a CIA plot to assassinate the North’s leader, would be “an act of war.”

The NSA’s success in getting into North Korea’s systems in recent years should have allowed the agency to see the first “spear phishing” attacks on Sony — the use of e-mails that put malicious code into a computer system if an unknowing user clicks on a link — when the attacks began in early September, two US officials said.

But those attacks did not look unusual. Only in retrospect did investigators determine that the North had stolen the “credentials” of a Sony systems administrator, which allowed the hackers to roam freely inside Sony’s systems.

In recent weeks, investigators have concluded that the hackers spent more than two months, from mid-September to mid-November, mapping Sony’s computer systems, identifying critical files, and planning how to destroy computers and servers.

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“They were incredibly careful and patient,” said one person briefed on the investigation. But he added that even with their view into the North’s activities, US intelligence agencies “couldn’t really understand the severity” of the destruction that was coming when the attacks began Nov. 24.

In fact, when, General James R. Clapper Jr., the director of national intelligence, had an impromptu dinner in early November with his North Korean counterpart during a secret mission to Pyongyang to secure the release of two imprisoned Americans, he made no mention of Sony or the North’s growing hacking campaigns, officials say.

In a recent speech at Fordham University in New York, Clapper acknowledged that the commander of the Reconnaissance General Bureau, Kim Yong Chol, with whom he traded barbs over the 12-course dinner, was “later responsible for overseeing the attack against Sony.”