berlin german police arrest 29-year-old man say active member islamic state plot truck attack ice rink arrest year anis amri fail tunisian asylum seeker islamist link hijack truck drive christmas market berlin kill 12 people come security service warn grow number radical islamists germany consider attack ice rink schlossplatz karlsruhe police south-western state baden-wuerttemberg say add suspect german citizen whose name give dasbar w. end assess areas around karlsruhe castle september 2017 begin seek employment delivery driver without success police statement say 2015 suspect travel iraq fight islamic state receive weapons train work scout seek potential attack target city erbil police say return germany follow year leave iraq dasbar work islamic state germany produce propaganda videos proselytize convert online chat room police say early month germany security service warn number salafists followers radical islamist ideology rise all-time high 10,800 though number prepare mount attack order hundreds german police arrest allege islamic state militant ice rink truck attack plot