The monolith was removed by an "unknown party" sometime Friday night, the agency said in a Facebook post.

"We have received credible reports that the illegally installed structure, referred to as the 'monolith,' has been removed" from BLM public lands, the post said.

"The BLM did not remove the structure, which is considered private property."

The monolith was first discovered November 18 by officers from the Utah Department of Public Safety's Aero Bureau.

As a Utah helicopter crew buzzed over the southeastern part of the state looking for big horn sheep last week, sitting beneath them in the middle of the desert was a bright shiny monolith, the stuff of sci-fi sagas.

Pilot Bret Hutchings of the state Department of Public Safety recalled someone on board spotting the gleaming metal object firmly implanted in a small clearing set against the redness of the rocks and sand below.

“He’s like, whoa, whoa, whoa, turn around, turn around,” Hutchings told KSL-TV in Salt Lake City. “There’s this thing, there’s this thing back there. We’ve got to go look at it.”

After Hutchings set their helicopter down in the remote area, which remains undisclosed to discourage a rush of gawkers, they got a closer look, but found "no obvious indication" of how it got there, the public safety department said.

Images released by the department show the monolith coming up to the shoulders of a man standing on the shoulders of another after the discovery last Wednesday.

Hutchings joked about the object's possible other-worldly origins and the obvious parallel to the classic 1968 Stanley Kubrick film, but thought the answer was more down to Earth.

"I’m assuming it is some new wave artist or something, or somebody who just is a big '2001: A Space Odyssey' fan,” he said.

In the film, an alien monolith is a recurring symbol that appears to play a key role in man's evolution from the apes.

The U.S. Bureau of Land Management, which owns the land, declined to comment on its investigation into the matter.

But it issued a reminder on Twitter that "using, occupying, or developing the public lands or their resources without a required authorisation is illegal, no matter what planet you are from."