The South Korean government has concluded that North Korea was behind last month’s massive cyber attack that brought down the networks of major South Korean media and banks.

A joint civilian-government-military probe team announced Wednesday that it was the North's Reconnaissance General Bureau that orchestrated the March 20 cyber attack.

The investigation team analyzed 76 malicious codes and computer access records collected from the affected networks. It also examined cumulative probe results from North Korea's past hackings provided by the military and the National Intelligence Service.

The team said the hackers conducted infiltration and monitoring operations on computers and servers at targeted South Korean institutions, starting at least eight months ago, and found weak spots in the networks.

It noted that of the 76 codes discovered, only nine were for demolition purposes and the other 67 codes were for infiltration and surveillance, proving that the attack was carefully premeditated.

The team found that at least six personal computers in North Korea were used in planting malicious codes in financial institutes in the South through 1,590 connections since June last year.

So far, the investigators have also traced 25 access routes in South Korea and 24 abroad that the North used for the attack. Eighteen of the routes in South Korea and four overseas are identical with the Internet Protocol (IP) address the North has previously used in cyber attacks against the South since 2009.

Broadcasters KBS, MBC and cable news channel YTN as well as Shinhan, Jeju and Nonghyup banks were paralyzed by the March 20 attacks.

Broadcast transmission proceeded normally but production and administrative affairs at major media were dealt a serious blow. Disruption in bank services also inconvenienced customers.

The networks were restored relatively quickly but it took days to normalize the numerous personal computers due to their sheer number.

An estimated 48,000 servers, PCs, and automatic teller machines were affected in total.

The North's Reconnaissance General Bureau was launched in early 2009 after combining three agencies in charge of anti-South operations.

The North is believed to have been fostering hackers on the state level since the late 1980s to prepare for cyber warfare.

The cyber war unit under the bureau is staffed with 3,000 people, and they hack into foreign networks, steal classified information, and spread viruses.