In order to protect against frivolous impeachments based on mere partisan, political, establishment or other undue interests -- all impeachments can only be invoked in the Senate after some grand jury concurs that the particular charge(s) have merit. A local grand jury will suffice for charge(s) brought against an official at state or local levels. Else the grounds for impeachment will have to pass muster of an impeachment grand jury made up of regular citizens outside the federal government and outside D.C. as well.
We add the grand jury requirements since congress and the special or independent prosecutor/counsels are prone to consist of or being orchestrated by spurious interests. Needing clearance through a grand jury provides protection by allowing only impeachments approved of the more common citizens making up the grand jury and explicitly fulfills Amendment V on behalf of civil officers of the U.S. in regards to impeachment.
To more likely curtail never-ending fruitless investigations: Six months after invocation the evidence gathered so far will be reviewed by a periodic grand jury. If the evidence seems to have merit and leads are promising the investigation can continue for at most three more months until the next periodic grand jury review. Repeat until special grand jury declares evidence and leads are dead-ends, beyond scope or maximum time and/or funds reached.