The unnamed official, who is close to the investigation, blamed the system failure on "malicious" code, the BBC reported. The virus is being analyzed, he said.
South Korea's Communications Commission initially suspected the outages were caused by the latest in a recent series of denial-of-service attacks.
During the 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 about 2 p.m., the Yonhap News Agency reported.
The South Korean Defense Ministry's computer networks reportedly were operating normally as of late afternoon.
The National Computing and Information Agency, which oversees computer networks at government organizations, also said their systems were working normally.
CNN reported Shinhan Bank was able to get its system back to normal after about 90 minutes.
Woori Bank also reportedly came under an apparent cyberattack about the same time but was able to restore computer operations with the help of an internal system, Yonhap said.
CNN said Nonghyup Bank managed to disconnect some of its virus-infected computers and its main server was functioning, allowing Internet transactions to be carried out.
Yonhap, quoting officials, reported the broadcast networks and some stations remained out of service as of late Wednesday afternoon but live broadcasting was proceeding.