I'm honestly shocked that Harry did not use the captured DE's to network to others and so on until he had every single one of them.
Some flaying and filleting might be needed to convince them to talk without Veritaserum (but their minds would likely betray them to Harry anyway) but, what's a little spilled DE blood (Or a lot. Or All) for justice?
Given what most FF's (and some circumstantial canon) evidence for the requirements to become DE's (rape, torture AND murder), I'm a firm believer in the "Kill'em all, let God sort'em out" philosophy of dispensing justice to DE's.