In an interview by the Kölner Stadt-Anzeiger from 2016-12-29, the journalist/feminist Alice Schwarzer said (in German):
Dass vor 20 Jahren 70 bis 80 Prozent aller Vergewaltigungen in Köln von Türken begangen wurden, war der Polizei wohlbekannt.
My translation:
It was well-known by the police that, 20 years ago, 70–80% of all rapes in Cologne were committed by Turkish people.
Let’s leave aside if it was well-known by the police, and let’s assume that "20 years ago" is a ballpark estimate (so maybe a year between 1993 and 1999, or whatever seems sensible), is it true?