Mystery aerial sightings show no evidence of being alien spacecraft, US spy chiefs say.
But intelligence experts are confounded when it comes to the unusual phenomena witnessed by navy pilots in recent years.
The details emerged ahead of a highly anticipated US government report expected to be released to Congress by June 25.
Investigators found that the majority of more than 120 incidents over the past two decades did not originate from any American military or other advanced US government technology.
The findings eliminated the possibility pilots who reported seeing unexplained aircraft might have encountered programmes the US government meant to keep secret.
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Officials believe that some of the aerial phenomena could have come from a rival power such as Russia or China amid fears Moscow or Beijing may have been experimenting with advances in hypersonic technology.
If they proved to be foreign aircraft then it would indicate that other nations’ research in hypersonic technology had “far outpaced” US military development, officials say.
While the forthcoming unclassified report will present few other firm conclusions, senior officials concede the very ambiguity of the findings meant the government could not definitively rule out other theories.
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They include that pilots may have seen alien spacecraft but there is no evidence to prove that as such.
Senator Marco Rubio said the government report should only be the beginning of the US effort to look into mysterious flying objects.
He added: “Men and women that we have entrusted with the defence of our country are reporting encounters with unidentified aircraft with superior capabilities.
“We cannot allow the stigma of UFOs to keep us from seriously investigating this.
“The forthcoming report is one step in that process, but it will not be the last.”
