Many of San Francisco's Asian massage parlors -- long an established part of the city's sexually permissive culture -- have degenerated into something much more sinister: international sex slave shops.

Once limited to infamous locales such as Bombay and Bangkok, sex trafficking is now an $8 billion international business, with San Francisco among its largest commercial centers.

San Francisco's liberal attitude toward sex, the city's history of arresting prostitutes instead of pimps, and its large immigrant population have made it one of the top American cities for international sex traffickers to do business undetected, according to Donna Hughes, a national expert on sex trafficking at the University of Rhode Island.

"It makes me sick to my stomach," said San Francisco Mayor Gavin Newsom. "Girls are being forced to come to this country, their families back home are threatened, and they are being raped repeatedly, over and over."

Because sex trafficking is so far underground, the number of victims in the United States and worldwide is not known, and the statistics vary wildly.

The most often cited numbers come from the U.S. State Department, which estimates that 600,000 to 800,000 people are trafficked for forced labor and sex worldwide each year -- and that 80 percent are women and girls. Most trafficked females, the department says, are exploited in commercial sex outlets.

Relying on research from the Central Intelligence Agency, the State Department estimates there are 14,500 to 17,500 human trafficking victims brought into the United States each year -- but does not quantify how many of those are sex victims. Some advocacy groups place the number of U.S. victims much higher, while others criticize the government for overstating the problem.

"The number will always be an estimate, because trafficking victims don't stand in line and raise their hands to be counted, but it's the best estimate we have," said Ambassador John Miller, director of the State Department's Office to Monitor and Combat Trafficking in Persons. The CIA won't divulge its research methods, but based its figures on 1,500 sources, including law enforcement data, government data, academic research, international reports and newspaper stories.

Women trafficked for the sex industry are predominantly from Southeast Asia, the former Soviet Union and South America -- lured to the United States by promises of lucrative jobs as models or hostesses, only to be sold to brothels, strip clubs and outcall services and extorted into working off thousands of dollars in surprise travel debts to their new "owners."

Federal investigators say that even those who come to the United States with the idea of working as high-society call girls cannot imagine the captivity and the degrading workload they face.

"Human trafficking is a multibillion-dollar business. In terms of profits, it's on a path to overtake drug and arms trafficking," said Barry Tang, an Immigration and Customs Enforcement attache with the U.S. Department of Homeland Security in South Korea. "There's a highly organized logistical network between Korea and the United States with recruiters, brokers, intermediaries, taxi drivers and madams."

The United States is among the top three destination countries for sex traffickers, along with Japan and Australia. Once in the United States, traffickers most often set up shop in California, New York, Texas and Las Vegas.

It's an underground world, but in more than 100 interviews with federal agents, experts and sex trafficking victims in California and South Korea, a picture emerges about how international traffickers buy and sell women between Asia and the West Coast.

Overseas, the trafficker is usually a woman. She recruits from clubs, bars, colleges, pool halls and restaurants, said Deputy Special Agent Mark F. Wollman, who oversees San Francisco for U.S. Immigration and Customs Enforcement. Recruiters fill the want ads in papers and the Internet, targeting vulnerable young women with fake job offers for waitresses, models and hostesses in America.

Traffickers fly the women to Canada or Mexico, and walk or drive them into California. In Canada, they slip through Indian reservations off-limits to the U.S. Border Patrol, often at night, and sometimes along snow-packed trails.

In Mexico, the traffickers lead the women over the same treacherous desert paths worn down by migrants heading to "El Norte" for work. More women come through airport customs in San Francisco and Los Angeles, using fake passports and student or tourist visas made for them by their traffickers.

It's relatively easy for traffickers to evade authorities at the checkpoints -- land, air or sea -- because women still don't realize at that point that they are being tricked.

"It's not like the movies where you open a trunk and you interview them and they tell you everything," said Lauren Mack, special-agent-in-charge with Immigration and Customs Enforcement in San Diego. "They aren't going to tell you they're victimized because they aren't -- yet."

Once in California, the women are taken most often to Los Angeles or San Francisco, where they are hidden inside homes, massage parlors, apartments and basements, only to learn that the job offer was just a ploy. Typically they are locked inside their place of business, forced to have sex with as many as a dozen men a day. Sometimes victims are forced to live in the brothel, too, where five or six "co-workers" are crammed into one room.

Their "owners" confiscate their travel documents until the women pay off exorbitant sums. Often captors will ensure the women never pay off their debts, by tacking on fees for food, clothing or rent. Some fine the women for displeasing customers, being late to work, fighting or a host of other possible transgressions.

Yuki, 25, who fears for her safety and only gave her first name to The Chronicle during an interview in Seoul, said she was trafficked from South Korea to a karaoke bar in Inglewood (Los Angeles County), where she was assured that she would simply be serving drinks to men. Once there, she was ordered to sell $3,000 worth of drinks each month. When she failed, she was sent to the "touching room," a private suite where men could have their way with her for $400.

Sex slaves who work in massage parlors and bars are often locked in their place of business by double security doors, monitored by surveillance cameras and only let outside under the guard of crooked taxi drivers who ferry them to their next sex appointment.

Women report being beaten, raped and starved by their keepers. Kim, who also withheld her last name, told The Chronicle in an interview in South Korea that she was forced to pay $4,400 for plastic surgery to open her eyes and make her nose thinner and pointier, "like Marilyn Monroe."

Both women eventually escaped their captors and now live as shut-ins in Seoul, spending their time on the phone or the Internet or watching TV, too afraid to go outside and cross paths with someone from the network that trafficked them.

They are scared because sex trafficking rings are often run by criminal organizations that aren't afraid to use violence to protect the billions they generate.

Although it's not known how much money the San Francisco market generates for sex traffickers, federal agents confiscated $2 million in cash from 10 Asian massage parlors during a San Francisco raid in summer 2005.

Local police say the bust didn't make a dent in the illegal sex trade.

"The number of Asian massage parlors has doubled in San Francisco in the last two years," said Capt. Tim Hettrich of the San Francisco police vice unit. "Profits are huge. I have nine people working on this. I need three times that many to keep up."

There are at least 90 massage parlors in San Francisco where sex is for sale, according to the online sex Web site myredbook.com. The site has been around since 1997 and has more than 55,000 reviews of Northern California sex workers. It is used by johns, yet is also a main monitoring tool for law enforcement. On average, there are about eight women working in a massage parlor, police say. That would mean more than 700 Asian sex masseuses working in San Francisco, based on 90 illicit parlors listed on sex Web sites and on police interviews.

But the scope of sex trafficking in San Francisco is much larger -- women are also forced to work as escorts, outcall girls, erotic dancers and street prostitutes. Women are also placed in "AAMPs" -- Asian apartment massage parlors -- which are little more than apartments rented by traffickers who staff them with one or two sex workers. Business is done by word of mouth, and only customers approved by the owner are allowed in.

Police in Livermore, Concord, San Mateo and Santa Clara have all found residential Asian brothels in their neighborhoods in 2004 and 2005.

"There are thousands of trafficked women in San Francisco," said Norma Hotaling, who advocates for victims as director of the Standing Against Global Exploitation Project in San Francisco.

She can watch men come and go at all hours of the day to a massage parlor across the street from her office.

"In looking at the city, I've never seen it like this before in terms of the number of massage parlors. No one is going after the johns."

The city may even be unwittingly contributing to the problem. Thirty-seven of the erotic massage parlors on My Redbook's list have massage permits issued to them through the San Francisco Department of Public Health.

When asked about the city giving permits to illegal massage parlors, Johnson Ojo, principal environmental health inspector for San Francisco, said part of the problem has to do with a big backlog that was created when jurisdiction over massage parlors was moved from the Police Department to the Department of Public Health in 2004.

"We are catching up and inspecting each one," he said. "But prostitution is a police matter -- we are looking for health and safety violations. If we find anything suggesting trafficking, we talk to police."

When told by The Chronicle of the scores of erotic massage parlors with city permits, Newsom said, "We aren't doing our job. We should take these Internet lists and go down them one by one."

In July, Newsom waited with city inspectors one afternoon outside Sophia's Spa, an alleged brothel in an alley between an ultra-modern cocktail lounge and a sex shop on Geary Street.

A decoy, an Asian police officer in jeans and a T-shirt, stood in view of the security camera over Sophia's front door and pressed the buzzer. The metal security door opened.

He duct-taped the lock so Newsom, the inspectors, police, a social worker and a reporter could get in.

It was a rude awakening for the half-dozen men inside, one of whom was in the middle of a sex act with a masseuse on the lobby couch.

While sex between adults on the lobby couch indicates that Sophia's is not a holistic massage establishment, it's not a crime unless the police see money change hands.

Inspectors cited Sophia's for using the premises as a living quarters, for inadequate ventilation, for improperly attired employees and for using a bed instead of a massage table in one room -- enough to land the owner in a permit revocation hearing before an administrative judge.

Although Sophia's has a massage parlor license from the Department of Public Health, the establishment is emblematic of a booming Asian sex-trafficking business that operates with near impunity in the city, Newsom said.

It thrives because it's so hard to prosecute -- the same women who are needed on the stand to help win cases are the ones who are being threatened into silence by their captors, said Heidi Rummel, a former federal prosecutor with the sex trafficking unit in Los Angeles.

"We have to explain the woman's mind-set -- that she's often unsophisticated, comes from a country with a corrupt government and would believe her captors' lies that if she flees she could get arrested by police," she said. "Juries have a hard time. They wonder: If the door was open, why didn't she just run?"

Sex traffickers who get caught are rarely convicted of sex trafficking -- and they know it. It's a frustrating cat-and-mouse game for federal investigators and prosecutors, who spend a year or more keeping a sex slavery network under surveillance, and then none of the women held in captivity is willing to testify.

Local police face the same problems.

"Our undercover officers arrest women for prostitution weekly in the massage parlors," said Hettrich of the San Francisco vice unit. "We let her know if she cooperates with us, she won't go to jail. But she is more afraid of her traffickers than us."

Women are scared for good reason. Those who have become witnesses have been burned with acid, have disappeared, or have had their homes ransacked and their families harmed or threatened in their home countries, said Dong Shim Kim, head counselor at Du Re Bang (My Sister's Place), a shelter for sex trafficking victims in South Korea.

Newsom admits that city inspectors alone can't shut the illicit massage parlors down, but he says City Hall is starting to regulate an underground industry that it's never inspected before.

Newsom put together a team of health and safety inspectors in summer 2005, shortly after California's largest sex-trafficking bust -- Operation Gilded Cage -- made it clear that a lot of the sex in the massage parlors was not consensual.

City officials were taken aback that all 100 masseuses removed from the 10 parlors in San Francisco were Korean, just like the 45 others arrested statewide on charges of running an international sex trafficking ring. The federal case is pending.

Since the federal raid, just one of the alleged sex parlors targeted in Gilded Cage has been shut down by the city. Golden Dragon Spa was ordered to surrender its massage permit on Aug. 31, after a city administrative judge deemed the business a nuisance and house of prostitution. Two others targeted in the federal raid closed when their buildings were sold.

The city attorney's office has prosecuted several other massage parlors, but the punishments have been "sorely disappointing," said Julian Potter, Newsom's public policy chief.

A handful of problem massage parlors have been fined $2,500 and threatened with 60- and 90-day permit suspensions if more violations are discovered.

In a three-part series, The Chronicle will tell the story of You Mi Kim, a debt-ridden college student from South Korea who reveals how she was trafficked into sexual slavery in California.

Providing a rare glimpse into a shadowy world, "Diary of a Sex Slave" chronicles You Mi's harrowing journey from South Korea to Los Angeles and San Francisco.

This year, the U.S. government granted You Mi a special visa for trafficking victims, given only to those who can prove they were enslaved by "force, fraud or coercion." Now she hopes that by talking, she will help people understand that some Asian masseuses are the property of international traffickers.

THE SERIES

Today

Global sex trafficking is making inroads into the Bay Area

Sunday

"Diary of a Sex Slave," Part 1: Fooled by traffickers in South Korea

Monday

"Diary of a Sex Slave," Part 2: Trapped in Los Angeles

Tuesday

"Diary of a Sex Slave," Part 3: Trying to break free in San Francisco

E-mail Meredith May at mmay@sfchronicle.com.