I’ve seen those when I was at the Mandalay Bay a few weeks ago. They are very tall, reinforced solid concrete posts. I’m not sure what they were built for, maybe a sign. They have clearly been abandoned. That said, there is no easy way to ascend them without a crane, nor get down from them. Plus, a shooter should be highly noticeable up there from the venue.
Google Earth is about three years behind the present, so if you saw the towers as they are recently, then the site has obviously been abandoned.