North Korean computer hackers were behind an attack on the network of a South Korean bank last month, according to Seoul officials.
National Agricultural Cooperative Federation, also known as Nonghyup, suffered a systems outage on 12 April, meaning customers were unable to withdraw and transfer money or use credit cards for three days.
The Seoul Central Prosecutors' Office now says the problem was caused by an "unprecedented act of cyber-terror" from a group in North Korea.
In a statement given to Reuters, officials say the attack came from the same group behind hits on South Korean government and business sites in 2009 and this March.
An Internet protocol address used to break into the Nonghyup network was the same as one used two months ago for a distributed denial-of-service attack that came from North Korea.
The hack was an attempt to destroy the financial system which is "the backbone of (the South's) capitalist society" says the statement.