Following Toghtekin's death in 1128, his son, Taj al-Muluk Buri, became the nominal ruler of Damascus. Coincidentally, the Seljuq prince of Mosul, Imad al-Din Zengi, took power in Aleppo and gained a mandate from the Abbasids to extend his authority to Damascus. In 1129, around 6,000 Isma'ili Muslims were killed in the city along with their leaders. The Sunnis were provoked by rumors alleging there was a plot by the Isma'ilis, who controlled the strategic fort at Banias, to aid the Crusaders in capturing Damascus in return for control of Tyre. Soon after the massacre, the Crusaders aimed to take advantage of the unstable situation and launch an assault against Damascus with nearly 2,000 knights and 10,000 infantry. However, Buri allied with Zengi and managed to prevent their army from reaching the city. Buri was assassinated by Isma'ili agents in 1132; he was succeeded by his son, Shams al-Mulk Isma'il who ruled tyrannically until he himself was murdered in 1135 on secret orders from his mother,  Safwat al-Mulk Zumurrud; Isma'il's brother, Shihab al-Din Mahmud, replaced him. Meanwhile, Zengi, intent on putting Damascus under his control, married Safwat al-Mulk in 1138. Mahmud's reign then ended in 1139 after he was killed for relatively unknown reasons by members of his family. Mu'in al-Din Unur, his ''mamluk'' ("slave soldier") took effective power of the city, prompting Zengi—with Safwat al-Mulk's backing—to lay siege against Damascus the same year. In response, Damascus allied with the Crusader Kingdom of Jerusalem to resist Zengi's forces. Consequently, Zengi withdrew his army and focused on campaigns against northern Syria.