The three-sided metal structure was removed on Friday evening “by an unknown party” from the public land it was found on, the federal Bureau of Land Management’s Utah office said in a statement.

The bureau said it had not removed the monolith, which it considers “private property.”…

Officials said that the structure was most likely a work of art and that its installation on public land was illegal. It was unclear who had put it there — and when — but the art world quickly speculated that it was the work of John McCracken, a sculptor fond of science fiction. He died in 2011.

His son, Patrick McCracken, told The New York Times this week that his father had told him in 2002 that “he would like to leave his artwork in remote places to be discovered later.”