The Phoenix Lights Incident became a phenomenon from March 13, 1997, after thousands of people began telling of seeing huge triangular UFOs drifting over Arizona and the city of Phoenix.
The sizes varied from a Boeing 747 to multiple football fields.
Five lights were seen in a formation by thousands of people over three hours from 7.30pm to 10.30pm.
The sightings took place across a 300-mile area from Phoenix to Tucson.
It was blamed by the military as being flares set off during an exercise at the time.
So-called triangle UFOs are still seen in high numbers across the USA, according to the database of the Mutual UFO Network (MUFON), which keeps a global record of such sightings, but never before have they been seen in such numbers as during the Phoenix Lights Incident.
Many claim they are evidence of extra terrestrials visiting our planet, but other conspiracy theorists suggest they could be top-secret experimental craft developed by the US Airforce at places like the mysterious Area 51 military base in the Nevada desert.
The Phoenix Lights incident 20 years ago is still seen as unexplained by some UFOers.
The Paradigm Research Group (PRG) is a US lobby group campaigning to get an alleged “truth embargo” lifted by the US Government.
It is convinced that the government has evidence aliens have visited Earth but says such information is kept from the public amid fears of the effect it may have on religion and the rule of law.
The group is urging anyone who is not yet convinced that aliens are real and visiting us to watch a new documentary available on Amazon Prime called I Know What I saw.
Featuring testimonies from witnesses to the Phoenix Lights, and other UFO incidents, it also includes interviews with ex-military and even astronauts.
PRG founder Steve Bassett said: “March 13, 2017 marks the 20th anniversary of the Phoenix Lights event.
“For anyone who still has a doubt as to the reality of an extraterrestrial presence and a government imposed truth embargo, please watch this documentary.”
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Alien chasers claim no-one has explained what else the Phoenix Lights could have been.
It recently emerged at the International UFO Congress event that some people even experienced memory loss after seeing them.
A panel of UFO experts discussed the case as part of the conference in Scottsdale, Arizona, last month.
Several hundred people attended an event called The Importance of the Phoenix Lights-Mass UFO Sighting.
One element which emerged during the panel’s discussion was that there was a bizarre reaction to the sightings by a number of witnesses.
Triangle UFOs like those said to be seen on the night are still reported across the US.
Some reported a temporary amnesia-like state, when immediately after seeing the lights, they went blank or failed to discuss it with anyone.
Panel moderator filmmaker James Fox, who investigated the case to produce a documentary, said that during interviews with hospice workers, they told how they “watched the lights appear and disappear over a period of time but didn’t say a word about it”.
One woman told him: “We went right back to our tea.”
Jim Mann, director of the Arizona Mutual UFO Network (MUFON) branch, which investigates UFO and alien sightings, told of similar accounts.
Another witness told Mr Mann he stopped his car to watched the UFOs with several other people and “a craft slowly glided overhead”.
The man said: “Not a word was spoken. After it went on by, everyone got in their car and drove home.”
Many witnesses thought they saw a black triangle supposedly carrying these lights, but two witnesses who viewed them through a telescope identified them as a formation of high-altitude aircraft.
The panel also included Richard Dolan, a UFO writer and researcher, and Dr Lynne Kitei, a witness to the incident who went on to write the book Phoenix Lights: A Skeptic’s Discovery That We Are Not Alone.
They looked at other claims, including similar mysterious sightings which happened in the three months leading up to March 1997.
Witnesses to these earlier lights included Dr Kitei, she said.
The panel also heard that the mass sightings actually started outside of Arizona, azcentral.com reports.
They were first seen near Henderson, Nevada, and later in Mexico.
Mr Dolan said: “We call it the Phoenix Lights, but it’s really not completely accurate.
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“You’re talking about two distinctive types of events. Could be related. Could be the same thing. Could be something different.”
Despite the unanswered issues, some researchers remain loathed to say it means it must have been aliens.
Sceptics claim to have since proved that military flares, which were set off at the same time, were responsible for the multiple UFO sightings.
Many UFO experts, however, claim this does not satisfactorily explain all the accounts.
Mr Mann said: “Do we have evidence that it was an extraterrestrial event? No.
“We have evidence that it was an extremely bizarre event. We can’t put a label on it other than it was an anomaly.”
Some claim the Phoenix Lights could have been secret US technology like the alleged TR-3B craft.
Scott Brando runs website ufoofinterest.org which exposes hoax UFO sightings.
He says the Phoenix Lights Incident was explained as aircraft and military flares.
He told Express.co.uk: “The Phoenix lights were explained with two different events. The first was the flight of five planes in V-shape formation.
“This is what the witness saw in that day.
“The second event was the launch of military flares over Estrella mountains.”
Referring to triangle UFOs generally, he added: “About other similar sightings I’d like to see some reports by MUFON, however many formation of sky lanterns or the launch of skydivers.
“Golden Knights were often mistaken for UFOs.”
Nigel Watson, author of the UFO Investigations Manual, said the Phoenix Lights were actually the result of aircraft lights.
He said: “The navigation and landing lights of aircraft can easily generate UFO sightings.
“Aircraft are usually identified as such by their engine noise, but loud background sounds or the distance of the UFO and wind direction can hide them from the witness.
“At night, the lights of a large tanker aircraft re-fuelling several small jet aircraft can look like an impressive formation of UFOs.”
Referring to the lights, he added: “Many witnesses thought they saw a black triangle supposedly carrying these lights, but two witnesses who viewed them through a telescope identified them as a formation of high-altitude aircraft.”