I think a pro would have cleaned up the scent rather than leaving the weapon and ammo on site. But if a professional hit, he may not be found, may already be out of the country. In any event, if the money can be followed, we may not be shocked to learn where it originated.
TV and movies aside, you don't want to leave anything behind if you can help it because all of it, even if left behind intentionally, provides useful information to investigators. And if a pro actually intended to leave that rifle behind, they'd have left it right on the rooftop rather than increasing their detection risk by carrying it outside the building after using it.
Anyway, we're all speculating so I guess we'll see. I do think they'll catch the individual who took the shot. I'm not a conspiracist, but I do think there's a decent chance he blabbed about something like this before doing it. So some others may know. Possible there are some accessories before or after the crime.