U.S. Bancorp reported solid financial results for its fourth quarter, although it once again described how revenue gains had been at least partly offset by the additional costs of people hired "to support business growth and compliance programs."

It has offered the same explanation in filings going back into 2016 and 2015, too. U.S. Bank seems to routinely hire more compliance staff.

And that's just one reason it's baffling that U.S. Bank also just booked a $608 million expense to settle with regulators over compliance failures, a pending deal that seems to also include a formal deferred prosecution agreement.

Paperwork errors or late reports don't lead to that kind of settlement. Some broken compliance process must have gone unfixed for a very long time.

Shareholders of Minneapolis-based U.S. Bank already knew regulators were digging into this, as the bank first disclosed 1½ years ago that it was being investigated over what it called "certain deposit accounts" controlled by Kansan Scott Tucker. He's a colorful entrepreneur in the payday lending business who just got sent to jail.

The problems occurred in the bank's efforts to monitor and prevent money laundering, a catchall term for activities meant to hide where money really came from or where it's going. Bad guys do that to make the money in their pockets appear like the result of real business successes rather than crimes. Congress and regulators have put in place a series of rules since 1970 meant to stop it.

Law enforcement had plenty of tools already, but through these laws they have basically deputized banks and other financial firms in the fight against drug trafficking, terrorism and other bad acts. They have given bankers the responsibility to look over the shoulders of their customers.

The bankers didn't appear to ask for this job, and complying has only gotten more difficult as the financial system has gotten more complex and the expectations of regulators kept rising. The main federal bank regulator just listed, once again, anti-money laundering compliance as one of the biggest risks facing banks.

One thing banks must do is report all transactions using currency of more than $10,000. Criminals try to get around that by making sure any money moves in smaller pieces, a practice known as structuring.

If a customer made a $9,990 deposit every day for a week, you would have to admit that should strike someone at the bank as awfully suspicious. Policymakers thought of that, too, one reason financial institutions have to also file suspicious activity reports.

Yet another rule, being implemented in May, adds to banks' obligation to really know just who their customer is, including who really owns limited liability companies and other common forms of business entities.

Given that the settlement with regulators isn't yet final, U.S. Bancorp couldn't say much more about its compliance problem.

"One of U.S. Bank's key priorities is to maintain an exceptional [anti-money laundering] program that detects suspicious activities, and we are confident in the strength of the AML program we have in place today," a spokesman said in an e-mail. "U.S. Bank embraces the highest standards of integrity, risk management and compliance and remains committed to improving our controls and processes across the enterprise."

Maybe it's unfair to parse that statement and focus on "today," as compliance programs for all banks have been works in progress. The relationship with Scott Tucker certainly goes back to when the rules were looser. He reportedly came with the 1999 acquisition of Mercantile Bancorporation of St. Louis.

Even though that was a different era for compliance and the hindsight picture seems clear, it's still seems like Tucker should have raised some suspicions. Even a cursory approach to figuring out what Tucker was doing at the bank could have produced a file full of material.

A race car driver as well as entrepreneur, recent media stories described him being led directly to prison this month following his conviction on 14 criminal counts last fall related to his payday lending operations.

But there's so much more to read.

One particularly striking fact for any banker doing business with Tucker is that this wasn't his first trip to jail, having been imprisoned in the early 1990s for his role in more of a garden-variety bank fraud scheme.

A thorough look at Tucker by the Center for Public Integrity and CBS News six years ago found plenty of material. One story described timely campaign contributions to the candidate for district attorney in suburban Kansas City, followed by the reduction of a speeding ticket to a parking violation. The deputy sheriff who wrote that ticket in 2008 later found that surprising; Tucker's Mercedes-Benz had been clocked going 86 mph.

Even more curious, the reporters found, is that among the businesses that promptly donated to that campaign were two payday lending companies purportedly owned by the Miami Tribe of Oklahoma.

The close relationship between Tucker and the tribe turned out to be one way Tucker could have become a big money-laundering problem for a bank. Tucker was found, at trial last year, to have set up sham business relationships with American Indian tribes. He got the profits, only using the tribes to keep what he was doing from the eyes of state regulators. Left alone, he could continue charging 700 percent interest or even more for online payday loans that his companies made to poor people.

For U.S. Bank, the books are, thankfully, about to be closed on Tucker, referred to in the latest filings as a "former customer." Yet now we know one reason why the bank keeps hiring compliance people.

It takes a lot of work to make sure the bank doesn't open a checking account for the next guy like that.

lee.schafer@startribune.com 612-673-4302