Iraq

Guardian: American investigators probing anthrax outbreaks in Florida and New York believe they have all the hallmarks of a terrorist attack - and have named Iraq as prime suspect as the source of the deadly spores.

Senior US intelligence source:  Making anthrax, on its own, isn't so difficult.  But it only begins to become effective as a biological weapon if they can be made the right size to breathe in.  If you can't get airborne infectivity, you can't use it as a weapon. That is extremely difficult. There is very little leeway. Most spores are either too big to be suspended in air, or too small to lodge on the lining of the lungs.

They aren't making this stuff in caves in Afghanistan.  This is prima facie evidence of the involvement of a state intelligence agency.  Maybe Iran has the capability.  But it doesn't look likely politically.  That leaves Iraq.

Administration official:  We see this war as one against the virus of terrorism.

FBI email to US biologists: Jan 2002: The perpetrator might be described as 'standoffish' and likely prefers to work in isolation as opposed to a group/team setting.  It is possible this person used off-hours in a laboratory or (borrowed) equipment to produce the anthrax.*

* Indicates FBI was aware that Anthrax attack was likely homegrown terrorism as early as 3 months after mailings.