Phnom Penh, Cambodia (CNN) -- Aaron Cohen first met Jonty Thern and her older sister, Channy, in 2005 while singing in a karaoke bar in Battambang, Cambodia. He has come back to see them every year since.
The California native often schedules his trips for November, the month when Cambodians celebrate the Bonn Om Teuk water festival, marking the end of the rainy season.
"The whole country comes together for boat races. Hundreds of thousands of people descend on the waterfront and it's filled with colors and flags," said Cohen. "You know my thoughts about the water festival always include Jonty, because she and her sister would get a day pass during the festival."
There was a smile on his face when he started the sentence, but by the time he had finished, it was gone.
Cohen is a human rights advocate. He founded a charity called AbolishSlavery.org last year, but his work freeing victims of human trafficking began more than a decade ago.
At 6 feet, 5 inches (195 centimeters) with long, black hair, he stands out in almost every crowd. But Cohen often goes undercover to obtain the information needed for law enforcement officials to conduct raids and make arrests.
His trips have taken him around the world, from Sudan to Nicaragua to Israel. But, he says, in Southeast Asia the problem is especially bad.
"I would rank Cambodia right up there with India as one of the worst places in the world for sex-trafficking."
A bad problem getting worse
According to the NGO, End Child Prostitution, Abuse and Trafficking (ECPAT), as many as one-third of all sex workers in Cambodia are children. Government entities, including the U.S. State Department, are pressuring countries like Cambodia to do more to stop the modern-day slavery epidemic.
"We are making major strides in the fight against human trafficking. But it is a major problem, we know that," said Ambassador-at-Large Luis CdeBaca, who leads the State Department's Office to Monitor and Combat Trafficking in Persons. "You have estimates as to the number of people in servitude worldwide and it's anywhere from 12.3 million on the low end as cited by UN's International Labor Organization -- to as many as 27 million people on the high end. That's a number coming from the research done by (the aid organization) Free the Slaves. But 12.3 million is a baseline number that everybody agrees that there are at least that many people in forced labor, and that's far too many."
In its comprehensive 2009 Trafficking in Persons Report, the State Department put Cambodia on its Tier 2 Watch List. The ranking means the Cambodian government does not fully comply with the minimum standards for the elimination of trafficking, but is making an effort to do so.
"[In Cambodia] the number of victims is increasing and the number of prosecutions has gone down from the previous year," says CdeBaca. "The report shows that despite the overall effort, the government has not shown enough progress in convicting and punishing human trafficking offenders or protecting trafficking victims."
Cambodia is categorized as a destination country for foreign child sex tourists, with increasing reports of Asian men traveling to Cambodia to have sex with underage virgin girls. The State Department report states a significant proportion of trafficking victims in Cambodia are ethnic Vietnamese women and girls who are forced into prostitution in brothels and karaoke bars.
A chance encounter
Jonty Thern's short life could be a case study for that assessment. Jonty's family immigrated to Cambodia from Vietnam shortly after the Vietnam War.
Faced with gripping poverty and a debt, Jonty's mother sold her daughter, who was 10 years old at the time, to a person on Cambodia's border with Thailand.
That person told her mother Jonty would be selling flowers and candy to customers in bars and nightclubs. It was only later, the mother says, that she learned while there, Jonty would be repeatedly raped and beaten.
After three years of physical and sexual abuse, Jonty was released by her captors and allowed to return home to Battambang. Soon after, she and her sister willingly went to work at a karaoke bar to help the family pay off their debt, according to her parents.
The scenario in which Cohen describes meeting Jonty Thern, then 13-years-old, is as appalling as it is prevalent.
"I was working as an undercover sex vice," Cohen said. "I was posing as a sex tourist, going from karaoke bar to karaoke bar, massage parlor to massage parlor, looking for underage workers, to see if I could get them on camera soliciting me for sex."
As evidenced in the State Department report, it is a poorly-kept secret in Cambodia that many of these establishments are also operating brothels.
"I went to a number of karaokes and about my second or third karaoke of the night and I immediately notice this one really young looking girl. I requested Jonty and her sister and a group of other girls," Cohen said.
"In these bars, the girls are told to drink as much as they can, because they'll charge you for the beers. So this girl comes in and I noticed, man, she downed that beer in like two seconds. She seemed to be having a good time, she didn't seem unhappy or anything. But here she is nonetheless, a 13-year-old girl in a brothel drinking 10 beers in the time that I drank two," he added.
He said he invited several friends who work at a nearby victims' shelter to come join him. They posed as partiers as well, until Cohen felt comfortable to ask the manager an important question.
"After the girls began to dance and sing, I asked the mamasan what more can I get besides karaoke and so then she says 'well, for sex it's $50.'"
Cohen used the solicitation video from that night, recorded on a cell phone camera, to provide police with the information they needed to raid the karaoke brothel.
More than a dozen girls, including Jonty and her sister, Channy, were freed that night and sent to live in a victim's shelter, where they received counseling, care and an education.
Cohen's most recent trip to see Jonty and Channy in Cambodia was not a happy reunion. It was a trip planned so that he could say goodbye to one of them.
Three days before arriving in Phnom Penh for the water festival, Cohen and Channy, along with Channy's mother, spent the morning in an 8th century pagoda in Siem Reap, watching as monks conducted an ancient funeral ceremony. They were transferring Jonty Thern's ashes into a marble urn.
Jonty died of liver failure at age 17. Her family claims it was the result of years of alcohol and drug abuse she was subjected to while working first in the nightclubs as a 10-year-old, and then later in the karaoke bars.
"The ashes of my goddaughter are the symbol of why we have to do this. This doesn't have to happen. These girls do not have to be enslaved," Cohen said.
"We tried our best with Jonty and we failed because we lost her. But if there's meaning in her death, the meaning is that there is more work to be done. When I'm in that karaoke now, or when I'm in that massage parlor, she's my little angel. She's watching over me and she's protecting me," he added.
That evening, after watching the festival's fireworks display and saying goodnight to Channy, Cohen strapped an undercover watch camera to his wrist, and went to a karaoke bar.