[onetoo]

Being out of things to investigate is a reason to end investigation. It is possible to follow all leads to the end and still have no conclusive evidence.

[konart]

New leads can surface at some point though, no? Or do they commonly reopen investigation in this case?

[onetoo]

I don't know the exact procedures, but I imagine and would expect that they reopen the case if new substantial and substantive leads show up.