Unsure if you've ever progressed passed this point, but after resolving the goblin camp:
If you have the letter from the swamp, you have the opportunity to give it to Halsin, if you choose to save him and the grove. The option is there when you speak with him as he's confronting Kagha following the battle/clearing goblin camp.
... I've never had this happen, but that interaction might resolve the investigation quest since you're relaying the information?
