The first documented Muslim contacts with Bulgaria are dated to the mid-ninth century when there were Islamic missionaries in Bulgaria, evidenced by a letter from Pope Nicholas to Boris of Bulgaria that the Saracens must be extirpated. During the time of Tsar Simeon insignificant Islamic influences on Bulgarian art began to appear, though it is believed that these can be traced to Byzantine influence. Later during the 11th and 12th centuries, nomadic Turkic tribes such as the Cumans and the Pechenegs entered Bulgaria and engaged the Byzantine Empire. According to scholars, some of these were Muslim. Migration of Muslim Seljuq Turks to Dobruja during the 13th century is also mentioned.