At what point during the lead-up to the assassination of JFK did Lee Harvey Oswald first break the law (and could therefore legally be stopped)?
As far as I can tell, he purchased his rifle legally, he was allowed to possess that weapon (under the 2nd amendment). Bringing the weapon to his place of work with the rifle wrapped in bag of curtain rods wasn't unlawful. The book depository was his place of work so he wasn't even trespassing when he set up his 'snipers nest' and if he didn't discuss his plans with anyone else, then there was no conspiracy.
So until he squeezed the trigger he hadn't committed a crime? If a police officer has seen him with a rifle that morning, could he have (legally) stopped him?
(this question starts with the premise that the 'lone nut' hypothesis is correct. For the purposes of this question please assume it was)
Update - I understand that planning and/or intending to commit a murder is in itself a crime, but I'm more interested in what physical act he might do that was irrefutably a crime (without the need to delve into his mind). As I understand it, he could have been caught on the 5th floor cleaning his weapon and simply claimed he was preparing it for going hunting the following weekend?