Too bad; Trump knows who did it and that's good enough for me.
Based on your comment, you must also believe anyone who pleads the 5th must be guilty, right?
Trump never said the US was involved. He only said answering the question would get the US in trouble.
If it was any of our allies and we showed evidence it was one or more of them, the US would be in trouble from that.
If it was Ukraine and/or Poland and/or any of our European allies, the US would be in trouble by making that public.
What if it was Syrian rebels, and he is protecting them?
What if the US gave/sold the explosives and technology used but was never informed of the planned attacks?
Just because he claims to know the guilty party/parties doesn't mean it was the US.
What if he believes it was the group or nation he was told, but the Intelligence agencies lied to him? Would not be a fiurst.