I find it very suspicious that even before the intermediate investigation results were known, as evidence was still being collected, first US and then UK came out with statement that according to their sources it was a bomb...
I don't. Quite a few countries simply don't have government or private staff with training to investigate aircraft crashes, and call in the US to help.
As for your immediate suspicion, all it takes is the chemical analysis of explosion residue on items from the plane to state "bomb." That isn't exactly requiring any sort of "conclusion" of intermediate investigation results. It IS the conclusion.