It was an anonymous two-story house with an outdoor side staircase, nothing that looked ominous to Kevin Koliner when he passed by going to and from work. On one evening stroll, the federal prosecutor heard loud noises but figured it was just a party. Later, he'd discover the ugly truth.

In a squalid second-floor apartment, just blocks from the U.S. attorney's office, Mohammed Sharif Alaboudi ran a violent sex trafficking ring, preying on young, troubled women. He plied them with drugs and alcohol, gave them clothes and a place to stay, and forced them to engage in sex acts with strangers. Prosecutors dubbed his place a "house of horrors."

The case of Alaboudi, now serving four life terms, offers a glimpse into how the feds are waging an aggressive campaign to root out the illicit sex trade lurking in this seemingly unlikely locale: a low-crime state dotted with sleepy hamlets.

"We're just a friendly state and I think traffickers see this as a trusting place and think, 'They're never going to catch me. They're not so bright,'" says Jenise Pischel, program coordinator at Our Home Inc., a private non-profit that has helped trafficked girls, including a 14-year-old in the Alaboudi case. "Well, we seem to be catching an awful lot of them."

In recent years, the feds have pursued about 50 sex trafficking cases, three resulting in life sentences. Bolstered by state and local authorities, they're also getting support from Native American tribes, church groups and the Junior League.

The cases have ranged from predator stings at the last three Sturgis motorcycle rallies to busts of lucrative businesses that have transported girls as young as 14 to cities around the Midwest. Police also have detected a circuit some traffickers travel that includes the Bakken oil fields in North Dakota.

Most traffickers have been transplants with criminal records; two serving life were reputed Chicago gang members. Customers, or those caught in stings, have ranged from a Texas air traffic controller nabbed at Sturgis after answering a bogus online ad offering sex with a 12-year-old (his sentence: 15 years) to a Lamborghini-driving local doctor who prescribed illegal Oxycodone to a trafficker (his punishment: 22 months.)

While trafficking exists around the nation, there's something distinctive about South Dakota: About half the women in the federal cases have been Native American, a particularly vulnerable population.

"You've got poverty, you have high, high rates of sexual abuse, which is often a precursor to prostitution and you have just a sense of desperation on the reservation in terms of day-to-day life," says Sarah Deer, a law professor at William Mitchell College in Minnesota and an expert on domestic violence in Native American communities.

Native American women with drug or alcohol problems are especially susceptible, Deer adds. "It's, 'Come to Sioux Falls. Come to Rapid City. I'll make sure that you get the crack that you need. All you have to do is do some favors.'"

A broad coalition is tackling the problem. Federal prosecutors have trained tribal law enforcement at all nine reservations on how to identify trafficking. Police have led workshops for motel workers. The Junior League has spoken about trafficking at schools, PTOs and 4-H clubs, financed billboards and prepared TV public service ads.

Though South Dakota has a pastoral image, police and prosecutors say its remoteness, pockets of poverty and highway system attract traffickers.

Some have migrated here "to be a bigger fish in a smaller pond," says Brendan Johnson, former U.S attorney. "They found less competition than they would have in a larger community."

It was Johnson, now in private practice, who began emphasizing the issue about five years ago, knowing convictions could bring stiff sentences. The federal mandatory minimum generally is 10 to 15 years.

Brandon Thompson, who controlled about 20 young women — he met some by hanging out near an alternative high school — is serving a life sentence. He pleaded guilty in 2011 to sex trafficking and solicitation to murder a federal witness. He'd attempted to recruit a cellmate to murder two teenage girls who were part of the ring, Koliner says.

Thompson's approach, the prosecutor says, was typical for traffickers: Seek out young women, many from broken homes and with histories of substance abuse. Lavish them with gifts and attention, act as boyfriend or manager, promise them a way to earn a lot of money.

"These guys might be bad at a lot of things in life, but they are excellent at finding that girl in a crowd, spotting the Little Red Riding Hood," Koliner says.

The most notorious case involved Alaboudi. At his trial, four young women told graphic stories of how he prostituted and sexually abused them and threatened and physically assaulted them if they resisted.

All had chaotic childhoods. One, identified as SJ, then 14, was on her own because her mother worked long hours to pay the medical bills of her husband, who sustained brain damage from a bar fight. At sentencing, the girl described her descent.

"Did I want to prostitute my body away to strange men?" she said. "No. I wanted to be loved by someone. I wanted a male in my life to show me care ....This is how I thought I had to do it."

Prosecutors also have charged attempted traffickers who "shop" online, answering phony ads placed by undercover officers that offer adolescent girls.

The spotlight on trafficking extends beyond law enforcement.

A 14-bed shelter for trafficked women will soon open in the south-central part of the state. Pathfinder Center will be run by Wiconi Wawokiya, a non-profit on the Crow Creek reservation that helps abused children and victims of domestic violence and sexual assault.

Lisa Heth, the group's executive director, says even though the public knows more about trafficking, there's still resistance to having victims as neighbors. "They say, 'Oh, my God, they're going to bring in prostitutes. Our crime rate is going to go up.' But it's right there in these small towns," she adds, "and they don't want to see it."

Pischel says getting trafficked teens back on track is "not a sprint, but a marathon" with frequent setbacks.

SJ, the 14-year-old in the Alaboudi case, made much progress in 1½ years, Pischel says, but when the young woman, now 18, stopped by recently, she was pregnant.

"I wish I could say she was better," Pischel adds. "I worry about where that child will grow up.... I know (SJ) has got the skills. I can only pray that she falls back on them."

Koliner isn't surprised. He's encountered other women who, after their trafficker is arrested, end up in another operation.

Still, the prosecutions have made a dent, he says, noting recent victims and witnesses have talked about how easy it is to get caught and about stiff sentences.

"We're sending the message to the men who are doing this: 'Don't come to our state. Drive on. If you want to do this, drive on.'"