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South Korean investigators say they have no proof that North Korea was behind the attack. However, the outage took place as Pyongyang warned Seoul against holding joint military drills with the U.S. that it considers rehearsals for an invasion.

North Korea also has threatened retaliation for U.N. sanctions imposed for the nuclear test and for its launch of a long-range rocket in December. Pyongyang blames Seoul and Washington for leading the push to punish the North.

On Thursday, in a vein of typical bellicose rhetoric, North Korea’s military threatened to attack American naval bases in Japan and an air base in Guam, where nuclear-capable B-52 bombers took off earlier this week to join the drills in South Korea.

The Korean Peninsula has remained in a technical state of war, divided by a heavily militarized border, since the foes signed a truce in 1953. Over the past decade, the two Koreas have engaged in deadly naval skirmishes in Yellow Sea waters that both countries claim. And, increasingly, their warfare has extended into cyberspace.

In 2011, computer security software maker McAfee Inc. said North Korea or its sympathizers likely were responsible for a cyberattack against South Korean government and banking websites that year. The analysis also said North Korea appeared to be linked to a massive computer-based attack in 2009 that brought down U.S. government Internet sites. Pyongyang denied involvement.

Previous hacking attacks on commercial ventures have compromised the personal data of millions of customers. Past malware attacks also disabled access to government websites and destroyed files on personal computers.

Last year, North Korea threatened to attack several South Korean news outlets, including KBS and MBC, for reports critical of Pyongyang’s activities.

In recent days, North Korea’s Committee for the Peaceful Reunification of Korea _ a government agency that often targets South Koreans in its push to draw attention to reunification _ warned Seoul’s “reptile media” that the North was prepared to conduct a “sophisticated strike” if its negative coverage continued.

“If it plays out that this was a state-sponsored attack, that’s pretty bald-faced and definitely an escalation in the tensions between the two countries,” said James Barnett, former chief of public safety and homeland security for the U.S. Federal Communications Commission.

An ominous question is which other businesses, in South Korea or elsewhere, may also be in the sights of the attacker, said Barnett, who heads the cybersecurity practice at Washington law firm Venable.

“This needs to be a wake-up call,” he said. “This can happen anywhere.”

Timothy Junio, a cybersecurity fellow at Stanford University’s Center for International Security and Cooperation, said South Korea has worked to protect itself.

“Part of why this wasn’t more consequential is probably because South Korea took the first major incident seriously and deployed a bunch of organizational and technical innovations to reduce response time during future North Korea attacks,” he said.

South Korea also created a National Cybersecurity Center and Cyber Command modeled after the U.S. Cyber Command. Junio said South Korea’s anti-virus firms also play a large role in stopping hacking attacks.

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