Mission: Defend the USS-Constitution [2]

I already killed all raiders in this mission, but the quest is still not completed. The mission-marker refers to the inside of the USS Constitution. But there are also no raiders. One time I entered the USS Constitution a raider was glitching through the bottom and the robots killed him, but the quest is still not accomplished. So, how can I finish this quest? I can't reload the game to an earlier state, since I've got not such a save...