BERLIN — An American man who made threats against Taylor Swift on social media was detained ahead of her first concert in the German city of Gelsenkirchen and will be kept in custody until her shows there are over, police said Thursday.

The suspected stalker, a 34-year-old whose name wasn’t released, had a ticket to Swift’s concert at Gelsenkirchen’s Veltins-Arena on Wednesday, police said in a statement. They said that, because an initial investigation couldn’t entirely rule out a risk, he was detained during entry checks to the event.