Sept. 22, 2010 -- The first time "Katie" danced at a strip club, she was 13, and in the seventh grade.
Not long before, the Oregon girl had gone on an ordinary outing to a mall in downtown Portland. She met some boys there and they invited her to a party in the suburbs. At the party, Katie met an older boy she would begin to date.
"He bought me a lot of things, like shoes, outfits and purses and stuff like that," said Katie, of J, who she began to think of as her boyfriend.
Before long, J told her he was having financial trouble, and he asked her if she would she dance, "just once," at a strip club, she said. Katie, who had only ever worn high heels to church and on special occasions, soon was forced into working as a stripper and a street prostitute six days a week, she said.
Every year, at least 100,000 children are forced into prostitution in the United States, according to the National Center for Missing and Exploited Children. And Portland, routinely voted one of the most livable cities in the country, hides a dark secret: It is a national hub for child sex trafficking, police and victims' advocates say.
Children, young teens from all over the country, but mostly from the small towns of the Northwest, walk Portland's streets.
Katie's inauguration into the life was like that of countless other girls.
That first day at the strip club, she remembers looking out from the stage.
"It was gross," she told ABC News. "They're like older than my dad and grandpap and I just wanted to go home. "
She was told, "You can't go home, you have to finish your shift," she recalled. "And I was like, 'I don't work here,' and they were like, 'You do now.'"
"I realized I pretty much walked into a trap," she said.
Katie's is a classic case, said Sgt. Mike Geiger, who heads Portland's sexual assault detail. Cute young girls are being targeted by pimps.
"They look for them in the malls, the parks, on MySpace, Facebook," Geiger said. "(A)nd they look for them in the schools and walking downtown. They strike up a friendship with them."
Preying on Vulnerable Teens
Pimps fish for girls whose home lives are lacking in some way.
If a girl indicates she's not getting enough attention or is having some other problems at home, "They will use the information and become their rescuer,'' said Geiger of the pimps' strategy. "'I can fix all that,' the pimps will say. 'I can give you a place to stay. You're way too mature to be treated like that.'"
The pimp will then give the girl clothes and other things, as well as affection, said Geiger. "And then the trap has sprung."
Ruth Roberts says that's what happened to her daughter.
"I was a single mom working really late ... making six figures,'' she said. "My daughter was getting A's and B's. Playing basketball. ... She had a good life, a good family, she didn't need anything."
But Roberts' daughter, then 16, met a guy on Portland's subway system. They "dated" and he convinced to her run away and live with him, Roberts said.
Soon, said Roberts, "She was walking the streets."
Forced to stroll Portland's notorious prostitution area around 82nd Avenue, referred to as "the track," even in the bitter cold.
Often, though, according to Roberts, her daughter was "rented" out on Craigslist.com.
Roberts said she spent her life savings trying to find her daughter and rescue her from the streets and from her pimp.
But her daughter, Roberts said, was afraid to leave. Her pimp once handcuffed her to a steering wheel of a car while he disappeared into a house, Roberts said her daughter told her. The pimp returned covered in blood, changed his shirt, and they went out to eat.
"She thinks he killed someone," Roberts said.
It is this fear that routinely keeps girls on the stroll.
Katie obediently worked the strip clubs by day and the track by night. At just 13 years old, she often would get picked up by johns within minutes of walking "the track."
She said she lived with J and lived by his rules. Other girls would come and go, she said. She remembers one in particular who was about the same age as she was.
"He bought her," she told ABC News.
Pimp Tells Teen, 'There is No God Where You Live'
That girl would find a way to escape, and Katie said she dreamed of doing the same.
"I just prayed," she said. "I was, like, crying and then he came out and he was like, 'What are you doing?' and I said, 'Praying.' And he said, 'Didn't I tell you there is no God where you live?'"
Michelle Alston knows that desperation first hand. She said her mother was prostitute, and at age 13, she decided to follow in her footsteps.
"When you go out there you risk getting robbed, raped, beat,'' she said. "You're getting in a car with someone you don't know. You risk that. ... Do I think it's worth it? No, but at the time it's all I knew."
Even though she was only 13 when she started, Alston said that sometimes she would have to lie about her age, when the john wanted someone younger.
"That's what they liked,'' she said.
When she was 14, Alston was introduced to the Internet and decided to go into business for herself.
"I would post online, Craigslist, City Vibe, Arrows Guide,'' she said. More than a dozen girls worked for her, she said, and she would post pictures of other girls, along with information about them, their fee and a phone number.
At 17, she was arrested and charged with "compelling and promoting prostitution."
She was a teenaged female pimp.
"I don't like that term,'' she told ABC, saying that she preferred to think of herself as a "financial advisor."
"I'd advise them to put their finances in my hands," she said.
Alston served three years in prison. She was pregnant when she began her sentence and gave birth in prison to a baby girl.
Helping women and girls to escape prostitution is fraught with challenges, experts say. For one, they are often treated as criminals, though they are just children and many have been forced into sex work.
"They might say to you, 'I've lived through more than most 40 year olds,' but then they say 'Oh, and I really love Justin Bieber,'" said Esther Nelson, a victims' advocate in Portland.
Nelson said she now sees girls forced into prostitution who are coming from and going to all parts of the country -- Atlanta, Hawaii, Chicago, New Orleans.
The city of Portland is home to more than 100 strip clubs and massage parlors, making it, per capita, the largest legal commercial sex industry in the nation.
"We have a very vibrant sex industry that is considered legitimate," she said. "And when you have that, you find the exploitation of children, because the sex industry is built on the exploitation of innocent people."
Police say Nelson is witnessing the results of sophisticated human trafficking now run by gangs who've realized it's often easier and more profitable to move girls than it is to move drugs.
Girls Branded With their Pimps' Logos
Authorities have many troubling stories to tell of the ways in which the girls are abused and victimized.
"People have been gang raped, tied up in Saran Wrap," Nelson said. "We're talking about 13-year-old girls left for dead in their hotel rooms without water or food for days. Pimps put logos on their bodies and they're treated like animals."
Geiger said Portland is badly in need of more shelters to help give the teen prostitutes a safe gateway off the streets.
"Often they end up in a juvenile detention center with less resources than they deserve," he said.
More often than not, Nelson said, the girls return to the street.
Escaping the Life
Katie was able to escape her pimp after waiting weeks for a bed in a shelter.
Roberts' daughter is no longer working as a prostitute, she said. But the young woman remains fearful that her pimp will find her.
Alston, who now works at a restaurant, said she hopes the family profession stops with her. She plans to shelter her curly-haired daughter, who she said has her hazel eyes, as much as possible.
"I want her to be successful and happy,'' she said. "I just want her to have everything I didn't."
Watch the investigation tonight on "World News" at 6:30 p.m. ET and more on "Nightline" at 11:35 p.m. ET