In Edward Gibbon's "The Decline and Fall of the Roman Empire" Theophilus was Patriarch of Alexandria from 385 to 412 AD. During his reign the Temple of Serapis was converted into a Christian Church (probably around 391 AD) and it is likely that many documents were destroyed then. The Temple of Serapis was estimated to hold about ten percent of the overall Library of Alexandria's holdings. After his death, his nephew Cyril became Patriarch. Shortly after that, riots broke out when Hierax, a Christian monk, was publicly killed by order of Orestes the city Prefect. Orestes was said to be under the influence of Hypatia, a female philosopher and daughter of the "last member of the Library of Alexandria". Although it should be noted that some count Hypatia herself as the last Head Librarian.
Alexandria had long been known for it's violent and volatile politics. Christians, Jews and Pagans all lived together in the city...there was mass havoc as Christians retaliated against both the Jews and the Pagans - one of which was Hypatia. The story varies slightly depending upon who tells it but she was taken by the Christians, dragged through the streets and murdered. Some regard the death of Hypatia as the final destruction of the Library. Others blame Theophilus for destroying the last of the scrolls when he razed the Temple of Serapis prior to making it a Christian church. (Source eHistory Archive)
"The argument of this book is that the Greek intellectual tradition did not simply lose vigor and disappear. (Its survival and continued progress in the Arab world is testimony to that). Rather in the fourth and fifth centuries AD, it was destroyed by the political and religious forces which made up the highly authoritarian government of the late Roman empire. "