The Zodiac Killer makes reference to "Old Norse", which was a North Germanic Language. The suspect Henning Lagies was born in the city of Kiel, in the northern German state of Schleswig-Holstein.
Old Norse is a North Germanic language that was spoken by inhabitants of Scandinavia and inhabitants of their overseas settlements during the Viking Age, until about 1300.
Languages belonging to the North Germanic language tree are, to some extent, spoken by immigrant groups mainly in North America. Henning immigrated from North Germany to North America in 1956.
References: http://en.wikipedia.org/wiki/Old_Norse and http://en.wikipedia.org/wiki/North_Germanic_languages
LETTER FROM THE ZODIAC KILLER
Dear Mr. Editor,
Did you know that the initials SLAY (Symbionese Liberation Army) spell "sla," [the word "sla" is written in script] an old Norse word meaning "kill."