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https://ehistory.osu.edu/articles/burning-library-alexandria

...The final individual to get blamed for the destruction is the Moslem Caliph Omar. In 640 AD the Moslems took the city of Alexandria. Upon learning of “a great library containing all the knowledge of the world” the conquering general supposedly asked Caliph Omar for instructions. The Caliph has been quoted as saying of the Library’s holdings, “they will either contradict the Koran, in which case they are heresy, or they will agree with it, so they are superfluous.” So, allegedly, all the texts were destroyed by using them as tinder for the bathhouses of the city. Even then it was said to have taken six months to burn all the documents. But these details, from the Caliph’s quote to the incredulous six months it supposedly took to burn all the books, weren’t written down until 300 years after the fact. These facts condemning Omar were written by Bishop Gregory Bar Hebræus, a Christian who spent a great deal of time writing about Moslem atrocities without much historical documentation.

So who did burn the Library of Alexandria? Unfortunately most of the writers from Plutarch (who apparently blamed Caesar) to Edward Gibbons (a staunch atheist or deist who liked very much to blame Christians and blamed Theophilus) to Bishop Gregory (who was particularly anti-Moslem, blamed Omar) all had an axe to grind and consequently must be seen as biased. Probably everyone mentioned above had some hand in destroying some part of the Library’s holdings. The collection may have ebbed and flowed as some documents were destroyed and others were added. For instance, Mark Antony was supposed to have given Cleopatra over 200,000 scrolls for the Library long after Julius Caesar is accused of burning it.