It is an ongoing investigation, sex for hire, and Sioux Falls police have made several arrests.
The Minnehaha County state's attorney announced a few details about those arrests this morning.
Since then, we've been examining court documents filed in the investigation.
There's a stack of affidavits filled with explicit details of the investigation.
It started with a look into the adult section of the web site backpage dot com.
The site looks similar to Craigslist, with ads for roommates, real estate, and buy-sell-trade.
But some were using the site's escort section to sell sex.
Police took notice of a photo of what appeared to be an underage female.
Investigators contacted her and obtained text messages sent to and from the girl's phone.
Those texts led police to several men who sent messages asking to meet or pay for sex.
Minnehaha County state's attorney Aaron McGowan said "last night or yesterday afternoon, 15 of 16 different arrest warrants were served between 3pm and 7pm, October 30th, 2012."
"These charges stem from an ongoing federal investigation regarding sex trafficking here in south Dakota. these are specific acts that occurred here in Minnehaha County," McGowan added.
Fifteen men were arrested and are out on bond.
There was a sixteenth person in the investigation but was not arrested because they are out of state -- and out of the county's jurisdiction.
Two other men were arrested in a second part of the investigation.
They also solicited sex from the same web site.
The girls were underage...working for Brandon Thompson of Tea.
Thompson is serving a life sentence after pleading guilty in April of 2011 to sex trafficking of a child.