Given the high level of sophistication, targeted nature of the attack, and significant funding implied by the ongoing attack method, a nation-state actor is the most probable responsible party. Nation-state actors often engage in espionage and target sensitive government-related data. They possess the capabilities and funding to carry out advanced and persistent threats. An unskilled attacker is unlikely to have the required sophistication; a hacktivist would more likely be motivated by political beliefs and not typically target defense data; an insider threat would have access but may not have the same level of sophistication or require significant external resources.