Lee,

I would say that any "Open" investigation must have some confidentiality to it until it is completed and then action is either taken or not taken.  So my thought is that, regardless of FOIA, it would be no different in my opinion of a pending case with the police or similar to a journalist protecting its source until such time as disclosure is warranted or even appropriate, if ever.

The backlash of releasing names of complainants could be unwarranted harassment.  

I use an Affidavit form for written complaints so that the person swearing it out attests that they would testify about it in a court of law before I even investigate.

Maybe a form like this would make it more clear, if you don't already have one, that the complaint is part of an investigation the moment you receive it.

Have a great day!

Kim