Because T-visa holders can apply to bring family members to the U.S., officials warn of the potential for fraud. In fact, about 550 T-visa applications have been denied.
"It comes down to documentation: Can they in fact prove they were trafficked, not just smuggled?" said Bill Strassberger, spokesman for U.S. Citizenship and Immigration Services, which issues the visas.
In one recent case, an investigation into South Korean prostitution rings in San Francisco and Los Angeles ended in more than 50 indictments this summer. Investigators believe that the women in Southern California were not forced to work as prostitutes.
"It's just a matter of getting to the truth," said Kevin Jeffery, deputy special agent in charge of Immigration and Customs Enforcement in Los Angeles. "A lot of times the truth isn't what the public wants it to be. That's the dilemma."
It's not easy to determine who is a trafficking victim, authorities said. Traffickers often coach victims on what to say, or frighten them into saying nothing.
"We have to convince these people we are on their side," said Dina Romero, assistant special agent in charge at Immigration and Customs Enforcement.
That takes time and working closely with organizations such as the Coalition to Abolish Slavery and Trafficking, which provides shelter and social and legal services for victims in Los Angeles. Executive Director Kay Buck said victims are encouraged to go to police but are not pressured to do so.
"The ultimate decision rests with the client," Buck said.
For law enforcement, uncovering trafficking cases remains the biggest challenge.
Sometimes victims cannot escape because they are kept in guarded compounds, such as in the case of 71 Thai workers freed from an El Monte sweatshop in 1995. But more often, threats and psychological coercion keep victims from leaving.
"In the rare case, the victims are chained by physical restraints," said Assistant U.S. Atty. Caroline Wittcoff, who prosecutes trafficking cases in Los Angeles. "In trafficking cases, they are chained by their fear."
Proving that victims were compelled to work by force, fraud or coercion can be difficult because there often are no eyewitnesses. The crimes are also hard to corroborate because they often occur behind locked doors -- in private homes, sweatshops or massage parlors.
Nevertheless, 382 suspects have been charged in trafficking cases nationwide since 1996, according to the U.S. Department of Justice. Since then, 248 people have been convicted.
Most of their victims are poor, uneducated and unable to speak English, authorities said.
"The victims are isolated," said Rosa Fregoso, an attorney with the Legal Aid Foundation of Los Angeles who represented Khamphiranon. "Where are they going to find out about T-visas?"
No matter how isolated victims are, they often have some contact with the community. The new outreach efforts depend on that contact.
The Los Angeles Police Department recently received a $450,000 federal grant to train officers on how to identify potential victims and to set up a hotline and public service announcements. A special city commission is also heading an effort to educate building inspectors, firefighters, healthcare workers and other city employees, who may be the first people to encounter victims.
"They can be our eyes and ears," said Paula Petrotta, executive director of the Commission on the Status of Women, which is working with the federal government on the public awareness campaign. More than 60 city department heads and staff attended the first training session at City Hall last week.
Authorities believe there are many undiscovered cases in Los Angeles, in part because of its proximity to the Mexican border. They also warn that immigrants voluntarily smuggled into the country run the risk of becoming victims of trafficking. Immigrants may agree to work as waitresses, officials said, but then be forced into prostitution. Or they may agree to a smuggling fee, only to discover the debt can never be repaid.
"It's that deception, combined with the sort of general knowledge that there is better work elsewhere, that makes the victims very vulnerable," said March Bell, senior special counsel for trafficking at the Department of Justice.
"The recruiter is often a world-class salesperson who gains complete trust."
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T-visa is rarely used
Victims of human trafficking are eligible for a special visa that allows them to live and work in the U.S. for three years and apply for permanent residency. There are 5,000 available each year, but only a small number have been issued.
Denied: 550 (some applicants have been denied twice)
* As of Dec. 1
Sources: U.S. Citizenship and Immigration Services, U.S. Department of Justice