http://www.ling.helsinki.fi/uhlcs/LENCA/LENCA-2/information/kazan.html

The Kazan TATARS HAVE ALWATS KEPT ALIVE THEIR TRUE HISTORICAL NAME: THE VOLGA BOLGARS. Where do the Bolgars of the Volga come from? As early as AD 550, Gothic annals mention the Turkic tribes of the BOLGARS living to the north of the Black Sea. The powerful state of the Bolgars in the Black Sea area held out in the wars with Byzantium, but fell under the pressure of the Turkic Khazars, a vanished people who dominated much of Eurasia in the seventh century. From the descendants of these tribes who went wandering over Europe came the Balkan Bulgars, the Volga Bulgars, and even the Caucasian tribes of Balkars, Kumyks and Karachays. The founders of modern Bulgaria, today's Tatarstan and neighbouring Chuvashia were, in the fifth to seventh century, one and the same people.