It is unclear whether the parting ways by brothers was caused by the internal conflicts or strong Khazar pressure. The latter is considered more likely. The Bulgars led by the first two brothers Batbayan and Kotrag remained in the Pontic steppe zone, where they were known as "Black Bulgars" by Byzantine and Rus sources, and became Khazar vassals. The Bulgars led by Kotrag migrated to the middle Volga region during the 7th and 9th centuries, where they founded Volga Bulgaria, with Bolghar as its capital. According to Ahmad ibn Rustah (10th century), the Volga Bulgars were divided into three branches: "the first branch was called Bersula (Barsils), the second Esegel, and the third Bulgar". In 922 they accepted Islam as the official religion. They preserved their national identity well into the 13th century by repelling the first Mongol attacks in 1223. They were eventually subdued by the Mongols in 1237. They gradually lost their identity after 1431 when their towns and region were captured by the Russians.