The U.S. government was unable to determine whether more than 140 unidentified flying objects, many of them reported by Navy aviators, were atmospheric events playing tricks on sensors or crafts piloted by foreign adversaries, or whether the objects were extraterrestrial in origin, according to a long-anticipated report released Friday by the nation’s top intelligence official.

The report finds no evidence that the objects, characterized as unidentified aerial phenomena, or UAPs, were the handiwork of alien beings. But in almost all of the 144 cases that a team of government experts examined, a lack of data stymied their efforts to say definitively what they were.

The largely inconclusive results of the report, which was required by Congress, are sure to fuel Americans’ long-running interest in unexplained sightings, which have received unprecedented levels of attention in recent years from government officials and lawmakers.

The mere existence of the report is a remarkable acknowledgment that human beings have encountered objects that perform feats we cannot explain.

Footage from 2004 shows an encounter between a U.S. fighter jet and "anomalous aerial vehicles," which is military jargon for UFOs. (To The Stars Academy of Arts and Science)

“Some UAP appeared to remain stationary in winds aloft, move against the wind, maneuver abruptly, or move at considerable speed, without discernible means of propulsion,” the report found. “In a small number of cases, military aircraft systems processed radio frequency (RF) energy associated with UAP sightings.”

Observers reported these unusual movements and “flight characteristics” in 18 separate incidents. The task force analyzing the UAP incidents will now focus additional analysis on that small number of cases, the report said.

At only nine pages in length, the unclassified report is also likely to prompt public speculation about what information the government chose not to reveal.

Luis Elizondo, Former Director, Advanced Aerospace Threat Identification Program (AATIP) joins Washington Post Live on Tuesday, June 8 (The Washington Post)

NASA Administrator Bill Nelson said he had seen the classified UAP report when he was serving in the Senate. “The hair stood up on the back of my neck,” he said in an interview. He also spoke with some of the pilots involved in the incidents it documented. “They know they saw something,” he said.

While NASA was not involved in writing the public report, Nelson, who spent six days orbiting the Earth during a space shuttle mission in the 1980s, said​ he had asked the agency’s scientists to study the incidents that the report addressed and their potential explanations. NASA has a small office devoted to searching for extraterrestrial life.

Nelson said his personal view was that the vastness of the universe suggested there must be alien life. “If the universe is that big . . . is there likelihood of life? My answer is yes.”

Sen. Mark R. Warner (D-Va.), the chairman of the Senate Intelligence Committee, said in a statement that he was first briefed on UAPs nearly three years ago.

“Since then, the frequency of these incidents only appears to be increasing. The United States must be able to understand and mitigate threats to our pilots, whether they’re from drones or weather balloons or adversary intelligence capabilities. Today’s rather inconclusive report only marks the beginning of efforts to understand and illuminate what is causing these risks to aviation in many areas around the country and the world,” Warner said.

The report does find that not all the UAPs behaved in the same way, which led the experts to offer different hypotheses about what they might be. The objects “very clearly demonstrate an array of aerial behaviors,” said a senior U.S. government official who described the report’s findings to a group of reporters on the condition of anonymity because the document had not yet been released.

“There is no one answer about what these UAP are,” the official said.

The report offers five categories of potential explanations for the objects, which were observed between 2004 and this year. The first is, essentially, junk — man-made objects cluttering the air, such as balloons or even plastic bags, that are mistaken for craft. Only one of the 144 encounters was definitively characterized with “high confidence,” and it fell into this category. “In that case, we identified the object as a large, deflating balloon,” the report said. “The others remain unexplained.”

Then there is the air itself. Ice crystals, moisture or heat fluctuations could register as a flying object to cameras and sensors on aircraft or aboard ships at sea. Navy aircraft and vessels have tracked and, in some cases, recorded encounters with UAPs that appear to zoom across a field of vision at unexpected speed, which had led some experts to question whether the supposed objects were actually just tricks of light or water.

The third possible explanation, according to the report, is that the objects are some kind of craft designed by the U.S. government or an American corporation. But that explanation seemed unlikely, according to officials who had earlier been briefed on the report and were confident that the UAPs are not American technology.

The report’s authors noted, however, that they were “unable to confirm” that classified government programs account for any of the UAP cases they examined.

The objects could have been designed by a foreign adversary, the report found, offering a fourth explanation. China and Russia are making strides in hypersonic technology and directed energy, areas of increasing focus at the Pentagon.

But the task force said it lacked the data to indicate that the UAPs “are part of a foreign collection program or indicative of a major technological advancement by a potential adversary.”

The fifth and final category is one sure to entice ufologists and amateur sleuths, as well as U.S. officials: “Other.” The description is something of a catchall that could apply to encounters that were brief or generated too little data to advance a theory. Some of these encounters might be better defined with more information or through scientific advances to analyze the phenomena, the report finds.

The task force’s work doesn’t end with the release of the report. The Pentagon announced that it would develop a plan to formally carry on the mission, which will use artificial intelligence and machine learning, the report said, to recognize similarities and patterns in data from encounters.

There is now a formal process for Navy personnel to collect and report information about sightings, and efforts are underway to create a standard process across the military services and among government agencies, which should help generate more information for future study.

The report also found that the stigma around reporting encounters with UAPs, and facing ridicule for it, had hampered efforts to understand the phenomena.

Sen. Marco Rubio (R-Fla.), the vice chairman of the intelligence committee, said, “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. This report is an important first step in cataloguing 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.”