An interesting article in El Pais about the situation in the Girona suburb named Salt.
Salt was built back in the 70s to house migrant workers from the south of Spain during an economic boom in Catalunya. Eventually the workers became respectable citizens and moved to richer neighborhoods, leaving behind a vacuum that was filled by immigrants from Morocco, Senegal, Gambia, Pakisan and South America. Now the place is rife with crime, drugs, kids that don't go to school, etc. (I have to say, looking at the place with Google Street View, it doesn't look nearly as bad as the article says... I see a couple black people loitering around the "Productes Llatins" store)
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This kind of stuff happens all around the developed world and no one has really been able to deal with it well.
I think part of the problem is that European countries tend to throw up all kinds of barriers to keep immigrants from participating in civil society and then wonder why they don't integrate. In Germany, for example, Turks can now finally get citizenship, but are forced to give up Turkish citizenship in the process. Since this jeopardizes their ability to travel to meet their friends and relatives, they naturally refuse. The Germans think the Turks are disloyal, but the Turks just think they are being practical.
Arbitrary searches and harassment by the authorities doesn't help either. For example, at the Sant Cugat train station the other day, the police were stopping anyone that looked like they were from South America and demanding their papers. Some unlucky people probably got deported.
Will immigrants come to the police to report a crime in their community? Will the immigrant community prosper if criminals elements in the community are given free rein to victimize its members? Probably not.