CNN went undercover with the Mossos d'Esquadra - the human trafficking unit in the Catalonia region of Spain - as it tackled a major investigation into Chinese prostitution in the region.

Now, the unit takes CNN inside another case it has cracked. But this Nigerian prostitution ring had many factors investigators hadn't seen before.

Undercover police are on the frontlines of the fight against human trafficking and the Freedom Project has accompanied several agencies that have given CNN unprecedented access to ongoing investigations. A few months ago, CNN showed how the Mossos d'Esquadra - the human trafficking unit in the Catalonia region of Spain - solved one of Europe's largest forced labor cases.

This time, CNN takes a look at another major investigation into Chinese forced prostitution in the region.

Watch "Undercover Catalonia" on CNN International, November 24, London 0830 / Berlin (CET) 0930 /Abu Dhabi 1230 / Hong Kong 1630; November 25, Abu Dhabi 0930 / Hong Kong 1330; November 26, London 0730 / Berlin (CET)  0830 / Mexico City 2200 / New York 2300.  

By Misty Showalter, CNN

It's the moment the human trafficking unit with Spain’s Mossos d'Esquadra was waiting for: After months of exhaustive preparation and dozens of operations, it finally made the big bust.

This time, investigators broke up a Chinese prostitution crime syndicate ruthless in eliminating its competition - even expanding into an international human trafficking ring. Thirty-nine people were arrested, 17 brothels busted and 30 women freed.

The year-and-a-half-long investigation exposed a calculating group that within just two years wiped out any competitors in the residential brothel business, says Sub-Inspector Xavier Cortes, head of the human trafficking unit of Mossos d'Esquadra, the police agency for the Catalonia region of Spain.

(More: Mossos and unraveling the web of Spain’s sweatshops)

"[The syndicate] began offering a different product set, offering an exotic consumer [choice] of sex and very low prices compared to the rest of the market," Cortes says. "Others had to close and go because, evidently, the client dropped them. The [syndicate’s] prices were much lower and the product was innovative; it was exotic.

“Now that they've cornered the market,” he says, “they also want to recover the consuming market for women from African countries, and are trying to start to diversify the supply within the Chinese-controlled apartments."

The investigation began in June 2010 when detectives started to take a closer look at newspaper ads for Chinese brothels. In recent years, the Chinese population has exploded in Barcelona - the largest city in the Catalonia region where Mossos d'Esquadra operates. Barcelona has become a major trade route from China to the rest of Europe, so the increase in the population wasn't a surprise. But investigators began to notice there were more and more Chinese residential brothels - places of prostitution operated out of apartments - and fewer residential brothels run by any other nationality.

Prostitution is neither legal nor explicitly illegal in Spain. You can willingly prostitute yourself, but you cannot become a licensed prostitute. It's illegal to force someone into prostitution. Proving whether a woman is willingly selling herself, or is being forced into it, makes investigating prostitution very tricky.

(More: Spain’s hot spot for human trafficking)

Brothels are another issue altogether. Investigators say to avoid detection, many brothels try to work around the law by securing licenses to run as bars or restaurants. Women will work inside as waitresses, so that if any sexual activity does take place, they have receipts that show some other service was given.

But investigating Chinese brothels poses even more unique challenges. As Spain’s human trafficking unit knows from breaking up a previous Chinese crime ring involving hundreds of victims of forced labor, the Chinese community is very tight-knit, keeping most of its contacts inside its ranks and rarely communicating with authorities.

"How they network, the manner of hierarchically structuring the organizations, the manner of trafficking the different functions - they're different to those that we know already, of other organizations of the West," Cortes explains.

Detectives started to observe the Chinese brothels, sometimes trying to get inside posing as clients, and then once inside, announcing they are police conducting a routine check.

They would discover what they say is evidence prostitution was taking place: red lights, several mattresses on the floor in one room, condoms and toilet paper by the mattresses. These were no ordinary apartments. Yet the women inside would not admit they were prostitutes.

"Simply from the outside, [you] cannot see anything that catches your eye, just the door of a house,” says one of Cortes’ undercover agents. “Therefore the people who come here have prior knowledge, in other ways, that prostitution is practiced here - in this case Asian."

During the search of one brothel, agents took three women back to the police station. One of the women admitted she'd just come from China the week before. To investigators, this was proof that women were being trafficked from China to be forced directly into prostitution.

After more than 15 months of surveillance, police checks and phone wiretapping, investigators finally gathered enough evidence to make the bust, and on September 26, they moved into 18 suspected brothels at the same time, and another 15 businesses, nightclubs and homes on October 1.

Agents say what they found was not only proof that the sexual exploitation of women was the group's main objective, but also evidence that the organization was using Barcelona as a layover to traffic the women to highly profitable first world countries, such as the United States, Australia and Canada. Police are still investigating exactly where in those countries the women ended up.

"One of the most recurrent pathways that was being used at this time was the transfer of people trafficked from China to Turkey, to Istanbul with a tourist visa," explains Cortes. "Once there, they took the passport back in order to return them to their country of origin so there is no evidence that this person had not returned. They would then cross the border to Greece by land, where they were given new documentation with a new identity. Within Europe, the mobility is much easier. Once they arrived at the final destination, which was Barcelona, they were put into flats where they spent a period of two or three weeks without being able to go out on the street. From here they waited for their final documents with which they crossed the ocean."

The accused traffickers were able to do all this, detectives say, with a high quality counterfeiting document lab. Agents found hundreds of forged passports, holograms and even stamps to simulate visa entries into different countries. Police also discovered credit card forgery equipment, weapons that they suspect were used to threaten the women and even to extort other Chinese businesses in Barcelona, and a wide variety of drugs - more than 2,600 ecstasy pills, 400 marijuana plants, and the means to make a drug called Ketamine.

Of the 39 people arrested, 33 are in jail awaiting trial and six were freed pending further investigation. Mossos d'Esquadra is also issuing international arrest warrants for accused traffickers in China and elsewhere. The 30 women who were freed will be offered visas to stay in Spain if they cooperate with police.