Mark Ostrem thought prostitution had disappeared from Rochester.

Ever since authorities cleaned up and revitalized a once-sleazy block of town, there had been few signs of smut on the streets.

“Everybody in this community firmly believed – and I would have been in this camp, too – that it’s not going on down here,” said the Olmsted county attorney. After all, Rochester is the home of the Mayo Clinic, a tidy city full of world-class medical and computer professionals.

Then, a few years ago, police were getting calls from people suspicious about things going on in some houses. A few massage parlors, they noted, were staying open until 1 or 2 a.m.

Ostrem’s eyes were opened to the insidious nature of sex trafficking when a group of St. Francis sisters who had taken up the cause invited him to a seminar. He began to understand how trafficking works in the digital age.

Pimps take girls on circuits through Minnesota, stopping in towns such as Duluth, St. Cloud, Rochester and the Twin Cities. They use cellphones to create Internet ads while driving to a new town, do business there for a day or two, then disappear into the next town.

Ostrem began to realize his county was not immune to trafficking: “It’s rampant and it’s seemingly almost out of control.”

The customers are not out-of-town business people, he said. They’re local. In a January sting, nine out of 10 men arrested were from the Rochester area.

“It’s our hometown folks,” he said. “It’s local people and it’s going on right here.”

Ostrem became part of the cadre within law enforcement who view the girls being trafficked as victims. He is increasingly focusing his resources on arresting and prosecuting the pimps and johns.

In 2011, his office prosecuted six johns, nine pimps and 16 victims, offering most victims treatment alternatives to get arrests off their records.

Last year, the numbers changed to 18 johns, four pimps and two victims.

“No girl grows up saying ‘I want to prostitute myself, that’s my goal in life. I dream to do that,’ ” he said. “Nobody does that. Nobody goes into this thing willingly.”