Germany arrested three suspected Islamic State members from Syria on suspicion of preparing an attack on the city of Düsseldorf, the country’s top prosecutor said Thursday, detailing allegations of a long-planned plot that could further inflame the German and European debate over migration and security.

At least one of the three suspects came to Germany as a refugee, officials said. He was taken into custody at a small-town migrant shelter near the Polish border in a morning raid by heavily armed police commandos.

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