'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 ...
I think Trish is getting warmer.
"... we know very little about the library's history. That hasn't prevented historians over the centuries from proposing various scenarios. The three main suspects are Julius Caesar, Bishop Theophilus, and Caliph Omar. Contrary to myth, there wasn't one great fire that destroyed the library, but instead several documented fires over a span of centuries. It seems likely, then, that the destruction of the library was gradual. The problem is that we have few contemporary accounts, and later writers often have some axe to grind.
"The first significant fire was around 89-88 BC. Egypt was torn by war and civil strife under Ptolemy VIII, and much of Alexandria was burned ... Though never restored to its former greatness, the library was rebuilt and survived for many more years.
"The next fire was in 47 BC, when Julius Caesar and the Roman armies conquered Egypt. Caesar burned the harbor as part of this campaign. Seneca (3 BC-65 AD) says that 40,000 books were incinerated in this fire; others say less. We do know that many volumes were looted by Caesar's army and shipped to Rome ...
"The next fire came 300 years later, in 273 AD, when the Roman Emperor Aurelian invaded Egypt as part of his war with Zenobia of Palmyra. Much of Alexandria was burned, including the Brucheion district. Whether this fire destroyed the entire library or whether some portion was rebuilt is not known.
"As Christians gained dominance in the region, they felt uncomfortable with pagan temples full of pagan documents. In 391 AD, Theophilus, the patriarch of Alexandria, urged a mob to destroy the temple at Serapis, presumably at the same time destroying whatever books were left in the daughter library ...
"The final fire was in 645 AD, when the Moslem caliph Omar conquered Egypt. The story is that Omar was asked what to do about the books in the library, and gave the reply:'If the books agree with the Koran, they are not necessary. If they disagree, they are not desired. Therefore, destroy them.' According to tradition, the scrolls were used as fuel to provide hot water for the soldiers' baths for six months ..
"Whom do we blame for the destruction of the library? We like Matthew Battles' summary. He notes that scrolls (like books) erode and fall apart over time, and we're dealing with five or six centuries. If an old scroll were crumbling, a scribe would have to make a new copy by hand ...
"Battles suggests that the destruction of the library wasn't due to a single great fire, but on account of 'moldering slowly through the centuries as people grew indifferent and even hostile to their contents.'
"He concludes:'What happened to the books of Alexandria? Many, many centuries happened to them ...'"
Library: An Unquiet History, by Matthew Battles, 2003
History of Libraries in the Western World (4th edition), by Michael H. Harris, 1995