U.S. intelligence officials have no explanation for nearly all cases involving UFOs encountered by military pilots since 2004, according the findings of a highly anticipated report on the mysterious objects.

Of the 144 reported encounters with so-called unidentified aerial phenomena (UAPs) from 2004 to 2021, officials said they were able to identify just one with "high confidence," determining the object was a deflating balloon. The Pentagon task force said officials "currently lack sufficient information" to classify the other incidents with certainty.

Officials called on Congress to invest in standardized reporting and analysis of UAPs to aid efforts at reaching a conclusion on their origins. The report also found the objects could pose threats to flight safety and national security.

"UAP pose a hazard to safety of flight and could pose a broader danger if some instances represent sophisticated collection against U.S. military activities by a foreign government or demonstrate a breakthrough aerospace technology by a potential adversary," the report said.

Compiled by the office of the Director of National Intelligence, the report did not find any evidence to suggest the sightings were alien spacecraft or evidence of advanced technology possessed by a foreign power such as China or Russia. However, the report also did not rule out the possibility.

The report noted that observers saw "unusual UAP movement patterns or flight characteristics" in 18 of the incidents, including some cases where the objects moved "without discernible means of propulsion."

"For years, the men and women we trust to defend our country reported encounters with unidentified aircraft that had superior capabilities, and for years their concerns were often ignored and ridiculed," said Sen. Marco Rubio, R-Fla., vice chair of the Senate Intelligence Committee. "This report is an important first step in cataloging these incidents, but it is just a first step. 

"The Defense Department and Intelligence Community have a lot of work to do before we can actually understand whether these aerial threats present a serious national security concern," he added.

Officials have identified five categories that are likely to explain the incidents once they are resolved: airborne clutter, natural atmospheric phenomena, classified U.S. technological development efforts, foreign adversary systems and a catch-all "other" category.

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A portion of the report is expected to remain classified. Earlier this month, senior administration officials told the New York Times the classified material does not contain any information concluding the existence of alien spacecraft. They acknowledged that keeping part of the report hidden could prompt speculation otherwise.

Rep. Adam Schiff, D-Calif., chair of the House Intelligence Committee, said the report showed that encounters with UAPs are "not a rare occurrence."

"We should approach these questions without preconceptions to encourage a thorough, systematized analysis of the potential national security and flight safety risks posed by unidentified aerial phenomena, whether they are the result of a foreign adversary, atmospheric or other aerial phenomena, space debris, or something else entirely," Schiff said in a statement.