Story highlights It's the first time the South has officially accused the North of conducting the attacks

Seoul says the codes used were similar to ones used before by Pyongyang

The attacks last month infected thousands of computers at major companies

South Korea accused North Korea Wednesday of carrying out a wave of cyberattacks that paralyzed the networks of major South Korean banks and broadcasters last month.

An official investigation found that many of the malignant codes employed in the attacks were similar to ones used by the North previously, said Lee Seung-won, an official at the South Korean science ministry.

Although some observers said at the time of the computer crashes that they suspected North Korean involvement, this is the first time that Seoul has formally pointed the finger at Pyongyang.

The allegations coincide with a tense situation on the Korean Peninsula, with the North making repeated threats of war. South Korean and U.S. officials have warned that a North Korean missile test could take place at any moment.

South Korea believes North Korea had spent at least eight months preparing for the cyberattacks, which also affected hundreds of individual citizens' computers and websites that cover North Korea, Lee said at a news briefing Wednesday.

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There didn't appear to be any immediate reaction on North Korean state-run media to the South Korean accusations.

The main hacking attack took place on March 20, hitting more than 48,000 computers at the South Korean banks and broadcasters, authorities said.

It infected the companies' computer networks with a malicious program, or malware, that slowed or shut down systems.

The South's investigation found evidence including IP addresses and other elements used in the cyberattacks that it said proved North Korean responsibility.

The hackers routed the attacks through more 10 different countries, Lee said.

South Korea has accused the North of similar hacking attacks before, including incidents in 2010 and 2012 that also targeted banks and media organizations. Pyongyang has rejected the allegations.