Dozens of US citizens, perhaps as many as 50, could be detained in coming weeks in the wake of a child-sex trafficking sting that is shining a spotlight on the seedy side of Colombia’s booming tourism industry.

In mid-October, Colombian and US authorities, working with two non-profit groups, staged three simultaneous raids in the cities of Cartagena, Medellin and Armenia.

By the time the busts were over, 55 children, some as young as 11 and 12, had been rescued from sex traffickers. In addition, at least a dozen people were detained on allegations of trafficking of minors.

Those busts, however, are just the leading edge of a larger sweep that aims to jail US citizens who have been traveling to this Andean nation seeking sex with minors.

“We’re starting with concrete individuals that we know have already been here and already done it or are in the midst of their travel to come here for the second, third, fourth times to do this,” said Angie Salazar with US Immigration and Customs Enforcement and the US embassy’s deputy attache. “We don’t know how many (will be detained) yet because we’re still pursing all of those leads, but I would say probably in the 50s.”

Salazar said agents are following a trail of e-mail communications and monetary transactions made to Colombian individuals and brothels. Some of those busts are likely to be made in Florida, she said.

The October 11 operation was unique for the hemisphere because of the active role that non-profit groups played. While many organisations help pull minors out of the sex trade, members of the California-based Operation Underground Railroad essentially set up a law-enforcement sting.

The group contacted traffickers, played the role of tourists seeking underage sex, and organised simultaneous meetings in three cities for Colombian police to swoop in on.

Tim Ballard, the organisation’s founder and CEO, who led the bust in the beach resort of Cartagena, said he was offered four children - three girls and one boy, all ages 11 and 12. They were his for $1,000 apiece.

Mateo, another team member who asked that his last name not be used because he is still working undercover for the group, led the bust in the city of Armenia.

The team was following a tip that a man there had been producing child pornography for the past eight years. Mateo posed as the middleman for wealthy individuals “who had conquered everything in the US and this was the one thing they couldn’t do: have sex with minors,” he explained.

“The reason it worked is because this happens all the time,” he said.

The team rented a farm near the city to host the fictitious sex party, and the organisation brought in other actors to play the roles of the wealthy clients. For just over $2,000, their contact brought 16 girls to the party - about half of them underage.

Since it began operating in January, Operation Underground Railroad says it has rescued 230 trafficking victims in Ecuador, the Dominican Republic, Haiti and Guatemala. Beyond helping victims, the group’s goal is to take criminals off the street.

“The problem is everywhere, and it seems to be in every country,” Mateo said. “We’re not out to eradicate the whole thing; what we want to do is eradicate the impunity.”

Steve, a special agent with Homeland Security Investigations who participated in the bust and asked for anonymity for the safety of his family, has been working sex crimes for almost a decade. But he said the Colombian busts were eye-opening.

“When you see an 11-year-old boy who is thankful that the police are there because he was about to be sold into this trade, or a 13-year-old girl who thought she was going to a birthday party - those are heart-wrenching cases,” he said. “The volume of kids being offered was even shocking for me.”

Child prostitution is a global phenomenon, and Colombia is no exception. But as this country sheds its troubled past and emerges as a tourism hot spot, child trafficking is becoming an issue.

During the first five months of the year, international tourism, at 1.6mn visitors, increased 9.1% over the same period in 2013, according to government statistics. US citizens made up the largest share, accounting for 18.5% of visitors, followed by the European Union with 16.4%.