The Mongol invasion and destruction of Baghdad in 1258 constitutes not only one of the greatest catastrophes in the collective memory of Muslims, but also represents a paramount turning point in the Islamic history. Therefore, the introduction of an excerpt of a few pages length that appeared in some manuscripts of Juvayni’s Persian Tarikh-i Jehan Gusha, describing the fall of Baghdad to the Mongols, allegedly attributed to the famous Isma‘ili-Shi‘ite philosopher, mathematician and astronomer Nasir al-din Tusi, aroused considerable interest among the scholars. Motivating this interest was the lack of eye-witness accounts to this pivotal event in Muslim collective memory and history. Hulagu Khan’s official historian Juvaini’s Jehan Gusha covers the events until 1257, just before the conquest of the ‘Abbasid capital. Other contemporary works of historians are clearly not eye-witness, but rather later written accounts relying on other sources. Thus, a primary source of paramount importance on the invasion of Baghdad was to emerge via the eye-witness account of al-Tusi. Through the studies, translations and meticulous interpretive analyses of Boyle and Wickens in the 1960s, this primary source was introduced to academic scholarship. However, a close reading of the excerpt casts doubt on its authenticity. Most evidently, the account seems to refer to the Nizari Isma‘ilis as heretics. Many historical details are omitted in the account; evidently, an effort has been made to shape the collective memory relating to the fall of Baghdad via the authority of al-Tusi. This paper aims to re-evaluate the authenticity and the general historical features of the account attributed to al-Tusi through a critical interpretive approach.