South Korea has arrested two suspected North Korean agents for allegedly planning to assassinate the highest ranking communist official to defect to Seoul, the South’s intelligence service and prosecutors said on Wednesday.

The news comes at a highly sensitive time, as Seoul attempts to determine whether North Korea was behind the sinking of a warship last month which killed 46 people. The arrests suggest Pyongyang could be reverting to highly aggressive tactics that have been rare since the 1980s.

Seoul says two North Korean army majors defected to South Korea through Thailand, arriving in January and February. However, South Korean agents found inconsistencies in their stories and arrested them. Under interrogation, they allegedly said they intended to kill Hwang Jang-yop, 87, a former chairman of the Supreme National Assembly , North Korea’s most powerful lawmaking body.