An official investigation into a major cyber attack on South Korean banks and broadcasters last month has determined North Korea's military intelligence agency was responsible, officials said Wednesday.

The probe into access records and the malicious codes used in the attack pointed to the North's military Reconnaissance General Bureau as the source, the Korea Internet and Security Agency (KISA) said.

"It was a premeditated, well-planned cyber attack by North Korea", a KISA spokesman said.

"We've collected a lot of evidence to determine the North's Reconnaissance General Bureau led the attack, which had been prepared for at least eight months," he said.

A joint team of civilian and government experts traced the origin to six personal computers used in North Korea.

In order to spread malware in target computers, the hackers went through 49 different places in 10 countries including South Korea, the investigation found. The North had used 22 of the places in past attacks.

The March 20 attack completely shut down the networks of TV broadcasters KBS, MBC and YTN, and halted financial services and crippled operations at three banks -- Shinhan, NongHyup and Jeju.

It employed malware that can wipe the contents of a computer's hard disk as well as drives connected to the infected computer.

About 48,700 machines including PCs, automatic teller machines and server computers were damaged, KISA said.

The attack came days after North Korea had accused South Korea and the United States of being behind a "persistent and intensive" hacking assault that temporarily took a number of its official websites offline.

It also coincided with heightened military tensions on the Korean peninsula, following Pyongyang's nuclear test in February.