Jogger, huh? Presumably a jogger hauling around something toting a long gun, unassembled or not, would stick out, so at least 2 people?
If there’s a parking lot or street beyond third base the perp may have had the gun in his vehicle ... all he’d have to do is get it after asking about their identity. Or there was more than one.
Could be same person. Running suit, gun in car or something. Make two passes, first one unarmed.