It may be hard for Mainers to wrap their minds around the concept that the more people commit federal crimes, the more that can help keep their tax rates down.

But it’s the truth, nevertheless — and since fewer people are being detained for federal offenses in Maine, people’s taxes could go up. That’s the Catch-22 entangled in the ways by which the Cumberland County Jail pays its bills.

The jail was designed with excess capacity, able to hold more prisoners than the county’s law enforcement needs required.

As the saying goes, that’s not a bug, it’s a feature: The extra cells were added as a money-raising feature, because they were intended to hold people being held on federal charges because there is no federal penal institution within commuting distance of the federal court in Portland.

Those prisoners, who come under the jurisdiction of the U.S. attorney for Maine, not the local district attorney, must be paid for with federal funds, sort of like boarders at a rooming house.

One with bars, that is.

At any rate, each federal prisoner nets the jail $103 a day in room and board. And the figure even includes an amount set aside for overhead, to pay not just the costs in support and security, but for the overall costs of keeping the jail open.

Unfortunately, however, the average number of federal inmates has been declining, from 103.2 per day in 2007 to 56.7 so far in 2010.

Since the jail’s break-even rate is 101 prisoners daily, that has brought revenues down faster than a bad day on the stock market. Monthly income has dropped from $332,592 three years ago to $182,849 today.

And since jail operation is consolidated at the state level under recent reforms, everyone’s on the hook if lawmakers have to boost the current amount they provide for jail operations.

Sheriffs and state corrections officials have met with staff for Maine’s U.S. senators, hoping they can find out why the prisoner total is dropping. And the county has hired lawyers to negotiate a higher per-prisoner rate.

Crime doesn’t pay? Maybe for criminals, but when it comes to making ends meet, jail officials clearly disagree.