In a statement, the telco said it initially suspected a DDoS attack due to traffic overload but after it scrutinised the matter it found that the cause was a routing error.
KT added it would cooperate with the government to investigate the precise cause.
The telco is yet to announce what caused the routing error in the first place and how this led to the outage, which is expected to be announced at a later date.
KT's nationwide network suffered an outage on Monday for around 40 minutes at around 11am local time.
The telco's subscribers were unable to use their credit cards, trade stocks, or access apps, while some large commercial websites were also shut down during that period.
South Korean police, which is also investigating the matter, said it could not find any circumstances to indicate that there was an external cyber attack in its initial investigations.
Meanwhile, the Ministry of Science and ICT is still conducting its own investigations on the matter. The ministry has ordered KT to investigate the extent of the damage caused to customers by the outage.