Organized crime networks are making new connections as they jump into the lucrative business of smuggling migrants, the operations chief for Europe's top law enforcement agency said Thursday.

Fighting them has become a major preoccupation for European investigators, said Wil van Gemert, Europol's deputy director for operations.

"In trying to transport people from one place in southern Europe to the north of Europe ... organized crime groups are linking to each other. Part of them are responsible for the first transfer into, let's say, Germany, and then onwards to Sweden," van Gemert told The Associated Press. He spoke on the sidelines of a joint conference with Interpol that gathered law enforcement representatives from 50 countries to discuss how to join forces against the smuggling networks.

"We have to agree who is doing what and when," van Gemert said.

Advertising on social networks and web links, the networks promise safe passage to Austria, Germany, Sweden and Norway, said Gerald Tatzgern, head of Austria's federal unit combatting human trafficking.

"They show pictures, photos from dead children" and offer themselves as a safe alternative, he said.

The syndicates from throughout Europe, with backgrounds in drugs, money laundering or other illicit trade, cooperate in moving people northward. And, with no political agreement in Europe on how to cope with the human tide, the refugees see few options.

"They are so many that the human smugglers just open doors of cars for example, or lorries, and say 'If you don't pay me enough, go away. I have hundreds of thousands willing to be smuggled,'" Tatzgern told AP.

Juergen Stock, Interpol's secretary general, said law enforcement agencies are working with social networks, trying "to find new ways to limit the use of these social media as a facilitator also for criminal activity."

The criminal networks stretch as far as the migrants want to go.

"If you look at the investigations, it's not only about source countries or transit countries. It's also about destination countries. You have seen arrests in Germany, Denmark," van Gemert said.

On Thursday, Finland's Border Guard announced the arrest of 15 people suspected of trafficking migrants from Turkey to Sweden to Finland.