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Orchestrating the mass shutdown of the networks of major companies would take at least one to six months of planning and coordination, said Kwon Seok-chul, chief executive officer of Seoul-based cyber security firm Cuvepia Inc.

The company that provides network services for the companies that suffered outages said it did not spot signs of a cyberattack on its networks, said Lee Jung-hwan, a spokesman for LG Uplus Corp.

Lim said tracking the source of the outage would take months.

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Associated Press writers Sam Kim and Foster Klug contributed to this report.