LAST MONTH federal prosecutors officially disclosed a 17-month FBI undercover investigation of alleged corruption in local government contracting -- a far-flung operation of uncalibrated scope that was sure to arouse curiosity all around town and other places too. Where it will lead and what it might involve remain a large mystery, but clearly it started the clock running on what could be a lengthy and uncomfortable period for the capital city. Curiosity and a shortage of hard facts on the table soon generate suspicion, rumors and sweeping but groundless assumptions about the ethics of city hall in general and its leaders in particular. But what are we looking at so far? Is this investigation the start of something big -- or the beginning of an extra-large fishing expedition by federal authorities?

Answer: unknown at this time. How long until the federal investigators show their cards? This, too, is uncertain, but people with some expertise in these matters say it could be a year or more before authorities assemble whatever they decide to make as their case or cases. At this point, it is important to remember that so far there have not been any arrests, any indictments or any convictions since the investigation was disclosed.

What authorities do have now is a lot of material gathered from court-ordered wire taps, searches of various premises and an FBI "sting" operation. And what city officials have is a cloud of suspicion hanging over the Barry administration, a haze that won't lift until all of this is somehow sorted and officially resolved.

The use of a "sting" operation in itself raises questions about civil liberties and the quality of evidence that may have been gathered. The amount of time allowed to lapse before the prosecutors make or drop their cases also becomes an issue. At what point can Mayor Barry or other city officials reasonably demand that investigators put up or lay off?

It won't be enough for U.S. Attorney Joseph E. diGenova to carry on indefinitely without some kinds of status reports to the public -- status reports that should jeopardize neither the investigation nor the rights of those involved. For now, patience and understanding are preferable to gossip or conspiracy theories.