Events in Cologne and other western cities over New Year showed that security concerns about young immigrants from North Africa haven't gone away.

"They were trying to run rings around the German state," said Armin Schuster, security expert in Chancellor Angela Merkel's Christian Democratic Union party (CDU).

"The fact that so many people of the same background made their way to Cologne, just like last year - that was a test of strength."

Schuster said that Cologne's police had reacted appropriately to the threat, praising them for being "robust and decisive."

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