Nearly half of the servers of the Nonghyup agricultural bank crashed, destroying key data and leaving about 30 million customers unable to use ATMs or online services for several days, The Washington Post reported Monday.
"This was an unprecedented act of cyberterror involving North Korea," said Kim Young-dae, senior prosecutor in charge of the investigation.
Western analysts who studied the incident agreed the aggressor was probably North Korea and described it as the first publicly reported case of computer sabotage by a country against a financial institution in another country.
Researcher George Wicherski of U.S.-based McAfee Labs called the attack "Cyberwarfare 101."
"They are doing massive damage with simple means," Wicherski said.
U.S. officials say North Korea's growing expertise means that someday it could target the South's military networks, potentially endangering the secrets of close allies, including the United States.