FBI

This would be a very challenging job of investigative reporting, and indeed might win a prize for someone who succeeded in proving (to most people's satisfaction) who actually planned the attacks. The very widespread and varied media outlets made possible by the Web also partly work against pinning down such evidence, because they greatly multiply the potential for misinformation as well as for finding actual facts.

You can start with news databases such as Proquest National Newspapers (US only; in many large public libraries), or LexisNexis Academic (in large academic libraries) which lets you limit to Non-US as well as US sources; for example, if you wish to get perspectives possibly not at all shared by our own official sources. Limiting LexisNexis Academic to "Non-US Newspapers & Wires" gets 146 hits for the search: anthrax attacks and 2001 and theories. You can also skim through much larger sets, using LexisNexis' "Expanded List" display to show the hightlighted search words ("Key words in context") in sections from 25 articles at once; for example, the 589 hits for the search: anthrax attacks and 2001 and question. You can try to verify who might have reported a particular theory; for example 14 hits for: anthrax attacks and 2001 and "patriot act" - but bear in mind, that could include any mention of the FBI pursuing suspects using the powers granted to them by that act; conspiracy by either supporters OR opponents of the act, etc.
Compare also the 80 hits in "US Newspapers & Wires" for the search: anthrax attacks and 2001 and "patriot act". Proquest Newspapers seems to get fewer hits: 12 in "Citation and Document text" for the search: anthrax attacks and 2001 and question*, 7 for: anthrax attacks and 2001 and theor*. It would be tempting to look at all 502 for: anthrax attacks and 2001. But unfortunately, Proquest does not have a "Key word in context" brief display - you have to look at full text to see highlighted words.