Suspected North Korean hackers stole sensitive military secrets, including joint U.S.-South Korean plans detailing how to eliminate the Pyongyang leadership, during an attack on Seoul’s defense data system last year, a South Korean lawmaker said.

Rhee Cheol-hee, a member of the ruling Democratic Party, said in an interview with local media published on Tuesday that the hackers had broken into South Korea’s defense data center in September last year and snatched a trove of classified military documents.