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Officers detained five men after searching an apartment in Wesel, around 60 miles from the city.
Four were later released but a 30-year-old Tajik man remained in custody, with a court approving his detention until January 7.
Police said the man was taken into custody on Sunday evening and after questioning had obtained information that was relevant to state security.
"We are exhausting all legal possibilities to protect the people, the cathedral and the upcoming New Year's Eve celebrations," Cologne police chief Michael Esser said.
On Saturday, German police said they had received an alert that a group may be planning an attack on the Cologne Cathedral on New Year's Eve.
Security was stepped up outside the landmark building which is the largest Gothic cathedral in northern Europe, attracting some six million visitors annually.
German authorities have repeatedly warned of potential terror threats against the backdrop of the Israel-Hamas war and the repercussions beyond the Middle East region.