South Korea's military says a suspected cyberattack that netted secret defense plans may have been carried out by North Korea.

According to a Defense Ministry spokesman, the suspected hacking occurred late last month. He says a South Korean officer failed to remove a USB device when he switched a military computer from a restricted-access intranet to the Internet. The USB device is said to have contained a summary of plans for military operations by South Korean and U.S. troops in case of war on the Korean peninsula.