Handguns are banned in Canada....thus for the Killer to have brought one into the USA from Canada seems awfully suspect. How many events of smuggling would be required to evade all Security Measures and Guns Laws in Canada to make that happen.
Owing to the lack of Security Measures at a Domestic Baggage Claim in US Airports....very big odds the firearm was brought in from the local area by the Killer.
We shall soon learn this is a Self Radicalized Individual who was resident in the USA.....I am willing to bet.
Anyone care to bet the Firearm was legally purchased or was a Straw Man purchase and was bought in the USA recently.