Concludium
In our experience, there might be a Senate Investigation or a Royal Commission to look into a matter of national import. It might last several years or until everyone had forgotten the reason for its existence and then deliver an opinion that no one was to blame (at a cost of $130M/£78M)1. In Ankh-Morpork, the Patrician might invoke a concludium. This is a concerted effort by those of his clerks with the relevant specialties to examine evidence and come to a supportable conclusion.
The clerks are not paid per diem; there is only the suggestion that there is "no great rush". Conclusions are usually reached within a day.
An example of a concludium comes in Going Postal, when Vetinari orders an investigation into the finances of the Grand Trunk. The investigation is greatly aided thanks to the assistance of the Ankh-Morpork City Watch in recovering a little red ledger from the late Crispin Horsefry. It is not quite so aided when clerks from one of the banks financing the Trunk arrive, and attempt to make numbers add up differently if they stare at them long enough.
1 A recent example is that in the UK, where the inquest into the MPs' Expenses Scandal decided that the Treasury should recoup £1.12 million. The inquest cost £1.16 million. Well done chaps...