The Korean Communications Commission initially said the outages were the result of a cyberattack from an address in China but further investigation indicated a virus came a computer in South Korea's Nonghyup Bank.
"Malicious code seemed to be spread from the server [of Nonghyup Bank] and there were records of [it] being approached by someone at that time," Lee Jae-il, vice-president of Korea's Internet Security Agency told reporters.
Officials said they still believe the attack was somehow orchestrated from another country.
During the Wednesday outage, skulls popped up on some computer screens, which the Korean Internet Security Agency said indicated hackers had installed malicious code in the networks.
The South Korean National Police Agency said three broadcasters, KBS, MBC and YTN; three banks, Shinhan, Nonghyup and Jeju; and two insurance firms had reported their computer systems suddenly stopped Wednesday, the Yonhap News Agency reported.