Some random ideas...
If you look briefly at the Internet Archive, especially the Open Source Books
collections, you will see that there are many texts in Arabic. That could be largely
enough to make the FBI suspect that the Internet Archive is hosting some terrorist
propaganda. So rather than hiring someone who can read Arabic to check the content of the documents, they may find it simpler to decide that everything is terrorist propaganda, get all the information they can on those who have upoladed something written in Arabic, and interview all these gentlemen. If they declare having uploaded
the Arabian Nights, the FBI may then decide to ask someone to check the document.
This would be one rather obvious police method for a fishing expedition.
Looking again at the Open Source Books collection, you can also notice the presence
of a host of texts by characters such as Sir Oswald Mosley (http://www.archive.org/details/EuropeanSocialism), Adolf Hitler, Martin Bormann, Guido Von List, Lanz von Liebenfels etc... Besides these, one can also note texts
marked AAARGH written by Holocaust denialists. This huge mass of antisemitic documents may have attracted the attention of the FBI (who may suspect that
the antisemitic materials is uploaded by the same people who are uploading the
texts in Arabic).
Another possible explanation for such an expedition is simply that the Internet
Archive has a few organic chemistry/engineering textbooks that describe the
fabrication of explosives or combat gases.
Since the textbooks are not in copyright, the
explosives would not be RDX or hexogen, but the WWI explosives (Turpinite, Melinite, Tolite, Cheddite, Panclastite, etc...) would have a chance to be found in these books. Similarly, yperite and phosgen would qualify as combat gases.
The FBI may be trying to know who has downloaded some of these textbooks, hoping to nail some amateur terrorist in the process.
Of course, none of this is very reassuring since some people may end up on a FBI watchlist just for having downloaded an old organic chemistry book that happens
to contain a sketchy description of the fabrication of yperite.
But then terrorists hate so much the freedom of americans...