American officials investigating the UFO sightings by Navy pilots in recent years have not reached a definitive explanation for the unidentified aerial phenomena, or UAPs, as they've come to be known, three U.S. officials confirmed to ABC News -- adding a new layer of mystery to the interstellar drama ahead of an upcoming report on the matter.
Investigators compiling a report to Congress due later this month are said to have found no evidence to suggest the UAPs are linked to alien spacecraft, nor are they in a position to explain what has been reported by U.S. military pilots in encounters over the years, the three officials said.
One of the officials told ABC News that some of the dozens of incidents involving UAPs were able to be explained, for example as weather balloons or as atmospheric conditions, but other incidents were not able to be explained.
A fourth official said that investigators have also ruled out the possibility that UAPs are related to any secret U.S. military technology, or so-called "black programs."
At the Pentagon, a top spokesman declined to comment on press reports about what was included in the upcoming report.
"I'm not going to get ahead of a report that hasn't been filed to Congress, and I'm certainly not going to speak about intelligence issues here from the podium," said John Kirby, the Pentagon's top spokesman. "I've seen the press reporting, but I'm not going to be able to comment beyond that."
Kirby said the UAPs are taken seriously since safety concerns have been raised by incursions into aerial training areas where some of the incidents were documented. The spokesman also confirmed that Defense Secretary Lloyd Austin has been briefed on the progress of the report.
Meanwhile, weapons experts are casting doubt on the possibility that the sightings are hypersonic technology belonging to America's foreign adversaries.
These developments further muddle the list of possible explanations for UAP sightings, which have consumed the national conversation in anticipation of a report due later this month from the Office of the Director of National Intelligence and a Pentagon task force focused on the issue.
In the absence of answers, experts and former government officials have coalesced around two competing theories for the UAP sightings: they could either be extraterrestrial in nature, or they could be experimental military materiel belonging to Russia or China.
Mick Mulroy, a former deputy assistant secretary of defense and retired CIA officer, said that either way, "these need to be investigated."
"These objects appear to exceed our military capabilities," said Mulroy, now an ABC News contributor. "We need to determine who this is and what capabilities they possess. It is never a good thing to discover you are vastly behind in technology. From a national security perspective, we cannot presume benevolence – whether terrestrial in origin or not."
A New York Times report cited a U.S. official as suggesting that the UAPs could be Russian or Chinese military assets experimenting with hypersonic aircraft or weapons – advanced technology that would allow objects to fly at unfathomable speeds.
In recent years, both China and Russia have "accelerated" their hypersonic research and development programs, according to a report prepared by the nonpartisan Congressional Research Service in April. The race for hypersonic technology is high stakes, and experts fear the Chinese and the Russians are making progress.
"Open-source reporting indicates that both China and Russia have conducted numerous successful tests of hypersonic glide vehicles and likely fielded an operational capability," the report read.
Weapons experts, however, are wary of a hypersonic explanation for the UAP anomalies.
"I would be very skeptical about the hypersonic theory," said Pavel Podvig, a senior researcher specializing in advanced weapons at the United Nations.
Video evidence of the UAPs released by the Pentagon shows Tic Tac-shaped objects flying at superfast speeds and abruptly changing directions. Lt. Ryan Graves, an F/A-18 Super Hornet pilot, told the CBS News program "60 Minutes" last month that he saw the objects "every day for at least a couple of years" off the Atlantic coast.
David Fravor, another naval aviator, described a 2004 encounter he had with one of the Tic Tac-shaped UAPs off the coast of southern California.
"The Tic Tac's still pointing north-south, it goes, click, and just turns abruptly. And starts mirroring me. So as I'm coming down, it starts coming up," Fravor said. "And it's climbing still. And when it gets right in front of me, it just disappears."
Kelley Sayler, the author of the Congressional Research Service report, told ABC News that she is "not aware of any hypersonic weapon or aircraft that behaves in the manner depicted in the videos."
Podvig, the U.N. researcher, echoed that sentiment.
"Any object moving at hypersonic speed would generate a strong infrared signature, which doesn't seem to be visible in images of these UAPs," Podvig explained.
"It would also create a shock wave and sonic boom, which doesn't seem to happen in these images, either. Further, a hypersonic object would not be very maneuverable -- it can change a trajectory somewhat, but cannot do sudden turns, not to mention stops," Podvig said.
With no clear earth-bound explanation for the UAPs -- and with the recent dismissal of some previously accepted alternative theories – so-called ufologists have elevated the prospect that these unexplained sightings reflect extraterrestrial interactions.
Christopher Mellon, a former Pentagon official who has led the charge for further investigation into the UAPs, tweeted on Friday that "It is likely that there is not a single explanation for all of this."
For its part, White House press secretary Jen Psaki told reporters Friday that the report is "not in the conclusion phase, as I understand."
"I will say that we take reports of encouragement incursions into our airspace by any aircraft identified or unidentified very seriously and investigate each one," she said.
She said President Joe Biden had not been briefed on the upcoming report.