The First Crusade took Jerusalem by storm on 15 July 1099 after a five-week siege, and the killing that followed was extensive enough to shock even sympathetic Latin writers: the Gesta Francorum and Raymond of Aguilers describe streets filled with the dead, and Ibn al-Athir, writing much later, gives seventy thousand killed in the al-Aqsa mosque alone. Jews who had taken refuge in a synagogue were burned there according to Ibn al-Qalanisi. Benjamin Kedar's study of the whole source tradition shows the highest Arabic figures exceed any plausible estimate of the city's population, and Geniza letters from Egypt document Jewish captives being ransomed and Christian sources record survivors expelled or ransomed rather than killed to a man. The massacre was nonetheless a large-scale killing of a civilian population and became a fixed point in both Latin triumphalism and later Muslim memory of the crusades.
The armies of the First Crusade stormed Fatimid-held Jerusalem on 15 July 1099 and killed or enslaved much of its Muslim and Jewish population before establishing a Latin kingdom
Estimates for Fatimid Jerusalem in 1099 run from about 20,000 to 30,000, swollen by refugees from the surrounding countryside who had taken shelter behind the walls. Crusader letters claim tens of thousands killed and later Arabic sources up to 70,000; Benjamin Kedar's review of the evidence, which draws on a Cairo Geniza letter about the ransom of captives, suggests a far lower toll of some thousands, with many survivors enslaved or ransomed and the rest expelled.
The low end reflects modern minimalist readings constrained by the city's likely population and by evidence of ransoming; the high end approaches the whole non-Frankish population of the city. Ibn al-Athir's 70,000 and the Latin accounts of blood to the knees are rhetorical and are treated as such by Kedar and subsequent scholarship.
The killing of the civilian population and the burning of a synagogue with those sheltering in it are documented in the Gesta Francorum, Raymond of Aguilers, Ibn al-Qalanisi and Ibn al-Athir, and assessed critically by Benjamin Kedar.
Palestine, Egypt, Syria