The retirement fund for firefighters and police officers in Dallas is more than $5 billion in the red, and Mayor Mike Rawlings thinks someone should have to go to jail for messing things up so badly.

"As I have learned more in recent years and months about how the [DPFP] reached its current crisis, I have come to believe the conduct in question may rise to the level of criminal offenses," Rawlings said in a statement posted to his official Facebook page last week. "Anyone brazen enough to commit crimes that harmed those who sacrifice so much to keep our city safe must be brought to justice."

Rawlings said he spoke with the director of the state Department of Public Safety. The Texas Rangers confirmed to The Dallas Morning News they have launched an investigation of former pension fund administrators.

The Texas Rangers might be getting the second bite at the apple. As the Morning News pointed out this week, the FBI last year raided the offices of CDK Realty Advisors, which worked closely with the pension fund's former managers. The FBI doesn't confirm or deny investigations or targets, so the raid could be completely unconnected to the pension crisis.

A criminal investigation into pension mismanagement is unusual, but not totally unprecedented. Pension fund managers from California to Michigan to New York have been investigated and charged in recent years, usually for running some sort of self-enrichment scheme involving pension funds or contractors.

The difference in Dallas is that, so far at least, there's not any evidence of that type of corruption. That doesn't mean that the Texas Rangers or the FBI won't uncover such a thing, of course, but the problems sinking the Dallas police and firefighters pension plan have more to do with the structure of the fund than any overt criminality.

Put another way: even if the Rangers or the FBI find that there was criminal behavior happening inside the Dallas pension fund, it likely won't explain away the underlying, structural problems. Moody's estimates that the fund could be out of money by the mid-2020s, and that's not because someone on the inside is stealing cash from the vault.

The big problem for the Dallas pension fund is that benefits promised by city officials were too generous relative to what the city was contributing, and the investment returns failed to make up the difference. Add in a special provision that allowed members to withdraw their investments as a lump sum at retirement—which drains the fund of needed capital for future investments—and you have a formula for failure.

Dallas hasn't fully funded the pension system since 2009, and shortchanged it by $14 million this year, according to the Dallas Morning News. Now, an immediate infusion of $600 million—equal to 20 cents of every dollar the city spends in its current budget—to fix the city's pension problem, members of the pension board told the city council in May.

The problems aren't new, either. Dallas knew the pension fund was in trouble in 2005, when the city sold $535 million in bonds to refill the Dallas Police and Fire City Pension Fund. In essence, that moved the debt from one pile (the pension system) to another pile (the city's municipal debt obligations), in the hope that payments on that debt would be less, in the long term, than the cost of the pension system. Like in other places where similar pension obligation bonds have been used, it was a risky move—like "taking $535 million and throwing it on the dice table," said city council member Mitchell Rasansky at the time.

Internally, the pension fund made other mistakes. It overinvested in real estate (at one point, the Dallas police and fire pension plan had the highest percentage of real estate holdings of any major pension fund in the country, according to one report), leaving it vulnerable to downturns in the housing market.

If that poor investment strategy rises to the level of criminality is a question for prosecutors. What's really criminal about some public pension systems—and Dallas is hardly the only perpetrator here—is the gap between what some public sector workers get for retirement and what the average private sector worker does.

I'm thinking of situations like the one in El Monte, California, where two former city managers are getting pensions in excess of $200,000 annually while a quarter of the population lives in poverty and the average household income is just $32,000.

There won't be a criminal investigation into those arrangements, though, because they were legally approved by city governments. The most important difference between a retired city worker getting a $200,000 pension and a pension fund manager running a pay-to-play scheme is that one of those people is stealing from the taxpayers with the backing of city council.