Excerpts from Books and Wikipedia
Roman Emperor Theodosius, a Christian, ordered all pagan temples to be destroyed. In retaliation, the pagans killed Christians. When the Christian faction fought back, the pagans in Alexandria took hostages and barricaded themselves in the Sarapeum. During the course of the siege, some of the pagans may have damaged the Sarapeum. Later, Theodosius had the Serapeum destroyed. [Furtado: 1001 Days] "However, none of the accounts of the Serapeum's destruction mention anything about it having a library and sources written before its destruction speak of its collection of books in the past tense, indicating that it probably did not have any significant collection of scrolls in it at the time of its destruction." [Wikipedia]