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. FULL POST