If the charges were dropped by the prosecutor in Chicago can't a federal prosecutor file them? Not double jeopardy if it was dropped.
Sadly none of the actions with the actual alleged assault — I am not aware of a federal statute to prosecute. HOWEVER - the threatening letter he received - thru the US Postal Service could be prosecuted if it can be proven him or an accomplice sent it could lead to conspiracy. There is federal offense covering this sort of thing. 18 U.S.C. section 1038 makes it a crime to engage in any conduct with intent to convey false or misleading information that may reasonably be believed to indicate that a biological agent is present. It also makes doing so a civil action, i.e. allows someone to sue. Sadly - if we don’t see federal charges fairly quickly - I don’t expect any.