Strongly suggests against balloons - mainly because they already shot one down and it was no biggie. Aliens are not impossible any more, but drones of some kind remain the vastly most probable option.
It sounded like they are hovering at altitude which is hard to do. And they came from great distance. It would make more sense if they were triangle shaped and flying. Also, there was no balloon, they should be stealthed and not show up on radar.
My suspicion is that the descriptions have been of the payloads. They sound like balloon payloads. Maybe the balloon is low visibility. But doesn't explain why government doesn't mention balloons.
The outside possibility that isn't aliens is anti-gravity. I would strange that anyone who developed anti-gravity would keep secret and use it like this.
Drone at 40000 feet? That's higher than most airliners - that would need be one heck of a drone, especially given where they have shot it down and how far it would have had to fly.
Much more likely sounding/meteorological balloons - a lot of those are sent up every day, worldwide, with payloads that are roughly cylindrical in shape. And some of them can traverse huge distances, even across oceans, so such "UFO" doesn't need to originate locally at all.
NASA’s XL-Calibur travelled over 4,000 km between Sweden and Nunavut in 5-7 days[1]. Montana to Huron is less then half the distance. 24 hours is a bit fast, but not impossible.
EDIT: To give a more clear example. It took XL-Calibur a little more then 30 hour to travel from Hudson Bay to Yellowknife. That is not much shorter then some distance between Montana and Huron
The jet stream currently has a strong west to east north east component from Montana to the Great lakes. The winds peed varies greatly depending on altitude, but a floating object can easily be pushed along at speeds in excess of 120mph.
My suspicion is that the descriptions have been of the payloads. They sound like balloon payloads. Maybe the balloon is low visibility. But doesn't explain why government doesn't mention balloons.
The outside possibility that isn't aliens is anti-gravity. I would strange that anyone who developed anti-gravity would keep secret and use it like this.