Alien conspiracy theorists believe the strange cargoes, which have been seen as recently as this month, are the UFOs of alien visitors that have been captured by the authorities and are on their way to secret storage.

Last week Express.co.uk reported how a stunned eyewitness watched open-jawed as a “UFO” was apparently seen being transported by a police convoy on the back on a truck at night near the Fort Leonard Wood area of Missouri on February 1 at 10.15pm local time.

The alleged sighting was reported by an unnamed man to the National UFO Reporting Center (NUFORC) in the US. 

It is about 240 miles from the Wright Patterson USAF base, which is where conspiracy theorists claim crashed UFOs, and even alien bodies, have been hidden away by the military. 

According to conspiracy theorists black projects are when the US military secretly develops experimental and classified fighter aircraft.

In a report to NUFORC, the man said: “I saw a line of police cars spaced out about a mile with no traffic coming southbound. 

“I noticed more police cars with lights keeping traffic held back in the distance about another mile and a semi with flatbed was in the centre. 

“As the semi (articulated truck) approached, I couldn’t believe that the semi was carrying what appeared to be a saucer shaped object.”

Then, Scott C Waring, editor of ufosightingsdaily.com, said that video footage had emerged of the most recent sighting reported to NUFORC, from another source.

But there was confusion after it emerged the video may have been taken a year ago.

There have been other similar sightings around the US.

A US defence industry insider told Express.co.uk the sightings are actually evidence of secretly-developed technology by the US Air Force, in so-called Black Projects.

Andre Milne, is the founder of defence technology firm Unicorn Aerospace, which describes itself as “the world’s leading developer of advanced navigation technology.”

According to its website, Unicorn Aerospace is “committed to development of advanced aerospace technologies in order to maximise revenue stream growth potential into the respective markets of the private commercial and public military sectors.”

Mr Milne told Express.co.uk: “The semi circular UFO style aircraft being transported on the flat bed trailer in the video is in fact an advanced generation design of a long series of experimental aircraft called Flying Wing Bodies.

“In this design the main body of the aircraft is the actual wing of the overall aircraft itself.”

He provided images of “the original design prototypes that have been built for testing the aerodynamic properties of this experimental concept.”

He added: “I hope this helps clear up the confusion about the occasional sighting of UFO type aircraft being seen transported on heavy trucks around the USA.

“Great to see that the aerospace industry is still actively conducting applied research into unconventional aircraft designs.” 

Alien conspiracy theorists, who have suggested they were UFOs, claim they are taken to top-secret airforce bases like Area 51 in the Nevada desert.

But, Mr Milne has already said that, instead of Area 51 storing crashed alien UFOs, it is where the experimental black projects take place.

Peter Davenport, NUFORC director, agrees.

He said: “We have heard about that object before, and we believe that it is some kind of space vehicle, used by the US Air Force.”

In an Express.co.uk poll responded to by 626 readers, 35 per cent believed that the UFO in the video was a secret US craft, while just 17 per cent thought it was an alien space craft.

Scott Brando, of hoax-busting website ufoofinterest.org, said the object on the truck in the video was an X-47B unmanned drone, which is a tailless jet-powered blended-wing-body aircraft capable of semi-autonomous operation and aerial refuelling.