In a series this week, CNN shows the struggle against human trafficking through the eyes of the investigators at Mossos d'Esquadra, the police agency for the Catalonia region of Spain.

They wear football jerseys, T-shirts, jeans and sneakers. They look like ordinary customers having a beer at the corner pub but that ability to blend in is also key to their role in the fight against human trafficking.

They are the men and women of an elite human trafficking unit in Spain's Catalonia region and they have to get key players in criminal gangs to trust them.

The region is a hot spot for traffickers. Barcelona - its biggest city and one of Europe's marquee tourist destinations - provides a cloak for traffickers who bring victims in on tourist visas.

Large-scale criminal organizations from Eastern Europe, Africa and China are setting up shop - bringing people into Spain under the guise of giving them jobs, then keeping their passports and forcing them to work in nightmarish conditions, either in prostitution or labor exploitation. (Read more about Spain's hot spot for human trafficking)

It has kept the Mossos d'Esquadra undercover unit, which is formally called the Central Unit Against Trafficking of Human Beings, very busy.

Sub-Inspector Xavier Cortes helped form the unit in 2007, and he says taking on the massive criminal organizations is a complex mission.

"Research techniques are different than a regular criminal investigation, such as the solving of a robbery," says Cortes, the ranking officer in the unit who doesn't work undercover and is comfortable with CNN revealing his identity.

"To investigate criminal organizations, what one cannot do is solve the crime. You have to locate and dismantle the organization... You get to know who the members are, how they live, how they interact, and how the organization is organized. Where the money comes from and where it goes - all this from many, many hours of analysis and operative work."

Another challenge, Cortes says, is infiltrating the groups to learn more about them.

"Considering in the vast majority of cases, the criminal organizations doing the trafficking of people that operate in Catalonia are organizations from foreign countries, it is almost impossible to get agents to infiltrate them."

That's where the use of informants comes in handy. The unit's biggest bust - a Chinese forced labor case involving 80 alleged sweatshops - came from two men who were fed up with their working environment and decided to come forward. One had been stabbed in the hand as he tried to collect evidence.

What started with a complaint by two men ended in a case so vast, it took three years to unravel the massive, tangled web of exploitation, and is only now going before the Spanish courts.

"We went to the location of the shops they told us that there were assets, confirmed the existence of these workshops and from here we began an entire series of steps that would lead us to see if, as stated, there were links between all these workshops," Cortes explains.

The process was complicated. Eighty workshops, all allegedly tied to each other in some way.

The human trafficking unit needed to find out who was managing which workshop, and what was really going on inside. They began surveillance, first taping outside the workshops.

"So that we could see what was the pace of activity - how many hours workers were inside or if they could leave or not leave the workshops," Cortes said.

Undercover agents were sent inside posing as clients looking for cheap labor. They say they found some workers wearing next to nothing while they sewed clothes - the conditions inside the factories unbearably hot.

After several months of evidence-gathering, a judge was satisfied the unit had enough to move in and free the victims.

In Mossos d'Esquadra's biggest-ever simultaneous raid, 900 officers moved on the 80 workshops.

The conditions they found were shocking. Mattresses leaned against walls, ready for the few hours workers were allowed to sleep before returning to work. Some beds were hidden behind bookcases. In the worst cases, the workers ate, slept, worked, and sometimes used the bathroom - all in the same room.

Cortes said the operation freed 450 victims.

"One of the most important things we could get was that out of the 450 people freed as local workers, about 40 percent of them told us, by declaration, they were subject to the payment of a debt and that debt had a condition that forced them to work in these workshops," he said.

Yet freeing the victims wasn't the end to this case.

Cortes and his team then had to prove how all of the workshops were linked - who was paying the bills, who was directly responsible for violating the human rights of the workers. And who was at the top of the whole organization.

Cortes explained the complicated process. "We could see that the head of this workshop was at the time the owner of the van utilized by four workshops to carry their clothes. But at the same time, the policyholder of that vehicle was responsible for this other workshop, and in the same way that workshop was responsible for paying the water of three workshops over in another area of the town."

After three years of research and evidence-gathering, 150 people were arrested in all. The case is now in the trial phase, and everyone accused has pleaded not guilty.

As vast as the Chinese forced labor operation was, there are even more egregious cases for the human trafficking unit.

Prostitution is the Catalonia region's biggest problem, and it's led to some of the unit's most depressing cases.

Cortes says he has seen women seven months pregnant, forced to stand and prostitute themselves for 10 to 12 hours on the side of the road with no access to sanitation.

The worse case Cortes has ever seen involved a girl found near death on the street. Investigators learned she'd been held in a room for two years, and had contracted hepatitis from constant, unprotected sex.

Her discovery led to the bust of an Albanian prostitution ring but it's the victims that drive Cortes and his undercover team.

They work all hours, sacrifice their home lives, and doggedly pursue evidence in long-running investigations.

It's a dedication that bonds the men and women together as they handle some of the toughest cases in organized crime.

With the traffickers quick to change their methods after each bust, the undercover officers have many more cases ahead of them, blending in, winning trust and securing convictions.