In this book of Serbian history we came upon this, relating to the Bulgar empire:
"...since it is impossible to follow further Serbian history without some knowledge of the Bulgars, let me now pause to consider that people.
The first point to grasp about the Bulgars is that, unlike the Serbs and Russians, they were originally not Slavs at all. Their early history is wrapped in considerable mystery, but we may say roughly that they entered the Balkan peninsula in the seventh century, as a Mongolian central Asiatic race, akin to the Huns and Turks. Of recent years, since Serbia and Bulgaria have become usually hostile and always suspicious towards each other, many Bulgarian writers have rejoiced to emphasize their people's Tartaric origin. Pure Tartars, however, they certainly are not. Once settled south of the Danube they accepted the language and customs of the Slavs amongst whom they found themselves. The old Bulgarian language disappeared and their present speech is pure Slavonic. They were converted to the Slav form of Christianity and they intermarried with the Slav race, so that in the West of Bulgaria, where the survival of the Slavs was most widespread, there is little difference between the Bulgar and his Serbian neighbour over the frontier."