Two alleged "non-human" alien corpses each belonged to a single skeleton and were not assembled, Mexican doctors have claimed.
The doctors have carried out a series of laboratory studies on the remains after academics, scientists and archaeologists argued they were part of a hoax.
Physicist and presenter Professor Brian Cox was among the critics - and claimed they were "way too humanoid" to be genuine.
The mummified specimens were displayed in glass cases as part of an official unveiling at Mexico's Congress last week in a hearing that stirred excitement among UFO enthusiasts.
Politicians were told the remains were found in the city of Cusco, Peru, and were estimated to be 1,000 years old.
The Mexico City event was spearheaded by journalist and UFO researcher Jaime Maussan, who testified under oath that almost a third of their DNA is "unknown" and the specimens were not part of "our terrestrial evolution", Mexican media reported.
"These specimens are not part of our evolutionary history on Earth," he said in his presentation to Mexican government officials and representatives from the US.
"They are not beings recovered from a UFO crash.
"Instead, they were found in diatom (algae) mines and subsequently became fossilised."
Prof Cox has been calling for a sample to be sent to the biotechnology company 23andme for independent verification that the specimens aren't aliens.
"It's very unlikely that an intelligent species that evolved on another planet would look like us," he said last week.
It is not the first time Mr Maussan has claimed to have made an extra-terrestrial discovery.
In 2015, he claimed a mummified body found near Nazca in Peru was that of an alien, but it was later found to be the remains of a human child.
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Academics, archaeologists, and scientists have said mummified remains that UFO enthusiasts claim are aliens are generally modified human bodies.
They say the rest, especially smaller ones like those presented in Mexico last week, are bodies assembled with animal and human bones.
However, the tests by Mexican doctors on Monday suggest they were from a single skeleton and not assembled.
Speaking about his alleged discovery last week, Mr Maussan told the Mexico City delegation the specimens had earlier been examined at the Autonomous National University of Mexico.
He said scientists used radiocarbon dating to gather DNA evidence and X-rays had shown one to have "eggs" inside.
Mexican politicians said last week that the the information had left them with "thoughts" and "concerns" and with the view to "continue talking about this".