A police investigation has found that North Korea hacked the computer networks of SK and Hanjin Group and retrieved a massive amount of documents, some of which contained sensitive defense data.

The National Police Agency’s Cyber Bureau announced on Monday that the North seized around 42-thousand-600 materials from SK Networks and Korean Air, a subsidiary of Hanjin Group, by hacking into the two firms’ computer networks earlier in February.

Among the stolen materials that the North seized from Korean Air are designs of medium-altitude unmanned aerial vehicles being developed by the South’s military, as well as drawings and a maintenance manual of the wings of F-15 fighter jets being operated by the U.S. military.

Data related to the status of Internet installations in South Korean military barracks were among the roughly 24-hundred materials that the North seized from SK Networks.

The South Korean military, however, asserted Tuesday that no important military secrets were included in the leaked documents, saying most of the documents stolen from SK Networks and Korean Air by the North are already open to the public.

The compromised data of the U.S. F-15 fighter jet is said to be simple specifications of the jet’s external design such as its length and width, and has no relevance to South Korea’s F-15K.

The military said that the core part of the design of the medium altitude unmanned surveillance vehicle was not leaked.

The police said the swift responses by the targeted South Korean companies and their cooperation with the police investigation helped prevent the North from inflicting significant damage on the South.

According to police, a total of 130-thousand computers in 17 SK Group subsidiaries and ten Hanjin Group subsidiaries, were hit by the North’s hacking.

Police found that the IP address used for the latest hacking was based in Ryugyong-dong in Pyongyang.