The FBI has shut down an observatory and a post shop near Roswell, and no one knows why

Tinfoil hats will be in short demand in the tiny US town of Sunspot, New Mexico - just down the road from infamous alleged Roswell UFO crash site - where an astronomical observatory and a post office have been closed.

A week ago the National Solar Observatory was evacuated and "temporarily" closed to both staff and the public as a "precautionary measure", a spokeswoman told local newspaper the Alamogordo Daily News.

She wouldn't say why, and the local police force have also reportedly been kept in the dark.

"The FBI is refusing to tell us what's going on," Otero County Sheriff Benny House said. "We've got people up there that requested us to standby while they evacuate it. Nobody would really elaborate on any of the circumstances as to why. The FBI were up there. What their purpose was nobody will say."

He says Black Hawk helicopters have been seen in the air above the observatory, which includes a telescope that towers dozens of metres into the sky.

The Sunspot observatory.
The Sunspot observatory. Photo credit: Getty

The town's post office has also closed.

"We were not told why, we were told just to be out of the area," US Postal Service spokesman Rod Sturgeon told Sky News.

Conspiracy theorists have rushed to fill the information gap with wild theories of grand theft, energy weapons testing and experiments in defending against a solar flare that could cripple the world's electrical networks.

Popular YouTube channel SecureTeam10, which uploads video of UFO sightings, suggested the observatory inadvertently snapped a photo of an alien craft.

Others on conspiracy-focused pages on Reddit and Facebook have proposed a serious mercury leak, or that an organised criminal group plans to steal valuable metals - but that doesn't immediately explain why the post shop has also also closed.

One person on Facebook page Conspiracy Kitchen noted the closures happened the same day infamous conspiracy theorist Alex Jones was booted off Twitter.

The observatory's social media accounts have been quiet since the authorities rolled in.

The FBI has ignored requests from various US media outlets for comment, including ABC, the New York Daily News and USA Today.

Ironically - or perhaps suspiciously - the sun is expected to unleash a massive solar storm this week, following the opening of a hole in the sun's corona, its outermost atmosphere.

Sunspot is about 200km drive from the spot where a suspected UFO crashed in 1947. The official story is that it was a weather balloon.