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.
Barcelona is one of the most popular tourist destinations in southern Europe and the largest city in Spain's Catalonia region.
The location's beauty and mild weather draw tourists, but traffickers like the region, too: Vacationers can become clients for prostitution and traffickers also use tourist visas to bring people into the country.
The human trafficking unit for the region's police agency, Mossos d'Esquadra, stays busy.
Xavier Cortes heads up the unit, which works undercover.
"The city of Barcelona, as a large city is the place where many groups come together, that is a place large enough to have internal distribution of territories," says Cortes. "There are parts of Barcelona controlled by Nigerian groups ... Within Barcelona there are also areas controlled by Albanians ... and now we are finding that the most extensive phenomenon is in an expanding phase and is now generating our most worry, is in the arrival of Chinese controlling prostitution."
Fighting these groups is one of the biggest challenges for the human trafficking unit. Investigating forced prostitution is very tricky in Spain: Prostitution is not legal, but it's not illegal either. Forcing someone to solicit themselves is illegal. A woman can sell herself but one can not become a licensed prostitute.