Officials from the police, firefighting service, the Korea Electric Power Corp. and the national forensic lab carried out the examination.
On Saturday, a fire broke out in the basement of the telecommunications company building and burned about 150 meters of fiber optic cables installed underground, incurring some 8 billion won (US$7 million) in damage with no human casualties.
The accident paralyzed the fixed-line, mobile and Internet networks for individuals and businesses using KT in Seoul's western districts of Mapo, Seodaemun and Eunpyeong, and some parts of Gyeonggi Province for most of the weekend.
The network disruption brought convenience stores, coffee shops and restaurants in the areas to a virtual shutdown for the day as it blocked credit payment systems, and they had to turn down customers or take cash only.
KT said as of late Sunday it had repaired about 63 percent of the mobile network and most of the Internet services, including credit card transactions.
The company said it will take one week to fully restore the network.
The mobile carrier announced it will waive affected customers' bills for the month and come up with measures soon to compensate small shop owners.
