There probably is some upper bound, but its very very long. If the grand jury returned an indictment in an ongoing investigation, you would have to give years to allow an investigator in a massive case like this time to work.
I do not know anything about this particular nerve agent, but it is hard to believe that an actor with nation-state resources could not synthesize this kind of chemical agent. Other sources identified it as a chemical that was known to be in Russian stockpiles in the 1970s.
WMD Level of Difficulty
Chemical→Nuclear→Biological(assuming you want it to be controllable)