I found out recently that regarding the Boston Marathon Bombers, the FBI never learned where they manufactured their bomb. Searches of any places even remotely connected with the bombers never turned up any evidence indicating where bombs were made, or by whom.
Tamerlan was killed, and Dzokar claims ignorance and that his brother did it all.
I don’t think that’s true. they were assembled at home. very simple instructions...didn’t have to be a rocket scientist