The ancient accounts by Plutarch, Aulus Gellius, Ammianus Marcellinus, and Orosius indicate that troops of Caesar accidentally burned the library down during or after the Siege of Alexandria in 48 BC [REF].
However, the editor of Plutarch's translation notes that the "destruction of the Library can have been only partial", and that it occurred specifically in the Museum built by the first Ptolemy (283 BCE) [REF].