Anthrax attacks.
Not possible either of the top suspects ever did it. Anthrax more weaponized than top KGB/FSB bioweapons samples, sent from different states at the same time, anthax samples were often slightly different showing different manufacutring. Fake letters were sent out before the attacks even began. Anthrax samples traced back to Government/CIA lab. Obama sealed the case with executive order and noone is ever allowed to reopen it. Top FBI investigators on the case at the time, today say believe a "patriotic group" was behind the attacks.