When did it happen that the existence of an investigation became an excuse not to answer any questions? I mean this question seriously; when did it happen?
It doesn’t make any sense; the one has nothing to do with the other. One might argue that one doesn’t want to jeopardize a prosecution, but (1) that doesn’t stop people from answering questions when they want to, and (2) in most of these cases (e.g., Benghazi) there’s no prospect of prosecution in the first place.
Whoever first used the ploy, it has certainly become the Obama administration’s go-to strategy for containing any scandal from Gunwalker to Benghazi: First, you tell a bunch of lies, hoping the thing will go away. If it doesn’t, you launch an investigation. For a year or more, as long as the investigation runs, you answer no questions. When the press finally moves on, you quietly close the investigation and issue a whitewash. Thereafter, if someone has the ill grace to bring up the subject, you say they are dredging up ancient history. (Carney: “Benghazi happened a long time ago.” Clinton: “What difference, at this point, does it make?”)
UPDATE (5/28): I had forgotten about this piece from last September (which I noted here), demolishing the idea that there is any legal impediment to releasing information when investigations are ongoing.