South Korean telecommunication operator KT suffered a nationwide network outage on Monday, causing widespread disruptions of various daily tasks based on connectivity. Virtual classrooms were down, food delivery orders were lost, and clinics couldn’t access patient data.

The network outage, which began around 11 a.m. and lasted for nearly an hour, was initially said to have been caused by a cyberattack. A local police station dispatched a team of cyber investigators to KT’s headquarters in Seongnam, Gyeonggi Province, just south of Seoul.

KT, however, restated that it appears to have been caused by routing errors, not by large-scale DDos attacks. A routing error can cause connectivity problems in the process of transmitting and routing IP packets for the internet.

The firm said that they are still looking into the exact cause and apologized for the inconvenience caused by a nationwide network outage. While the network was down, KT users suffered lost connection on internet and phone calls. Securities companies using online and mobile trading programs also experienced inconveniences.

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Stores using KT service for payment systems failed to process credit card payments and were forced to ask customers to make payments via account transfer or with cash instead. Students taking online classes were unable to participate. At around noon, the connectivity of both wired and wireless services was restored. But in some regions, phone calls still were not going through.

Around 14.3 million people rely on KT for internet services as of March 2021. In 2018, KT suffered a network outage after a fire broke at its Ahyeon branch in central Seoul. The incident led to internet and phone disruptions in nearby areas, including the Jung-gu, Yongsan-gu and Seodaemun-gu districts in Seoul.