[SunkenCiv]

The story is basically hokum, was popularized by the polemical “The Thirteenth Tribe”.

[rmlew]

Are you suggesting that Rabbi Yehuda HaLevi made up the whole story? Are you suggesting that the correspendence between Khagan Joseph and Hasdai ibn Shaprut was fake? Are you saying that they Schechter Letter is a fake? Are you saying that the Kievan Chronicle randomly called the Khazars "The Jewish Kingdom?"

http://www.khazaria.com/khazar-quotes.html

Koestler's claim that all or most Khazars became Jews is false. In fact it is refuted in the primary sources. But some Khazars did convert and according to Khagan Joseph, they intermarried refugees from the Caliphate and Byzantine Empires.