Bakasi is right. According to the BKA home website (found here), it began in Hamburg.

Quote
The Bundeskriminalamt dates back to March 1951. At that point in time, the ”Law on the Establishment of a Federal Criminal Police Office” came into force. A short time afterwards the ”Criminal Police Office for the British Zone” in Hamburg became the Bundeskriminalamt (Federal Criminal Police Office), abbreviated BKA.


I did not, however, check for current locations, which are available on that page with one click. Bad on me!

I'm going with the explanation that this is an alternate world where Hamburg kept an active BKA office and Horst Müller is Hauptkomissar there. (It's easier than correcting my text, kind of like a computer programmer fixing the data after the program has run instead of fixing the program. And if you think that doesn't happen, you're mistaken. I've seen it.)

Life isn't a support system for writing. It's the other way around.

- Stephen King, from On Writing