The panel's chairman, astrophysicist David Spergel, said his team's role was "not to resolve the nature of these events," but rather to give NASA a "roadmap" to guide future analysis.
NASA officials said several panelists had been subjected to unspecified "online abuse" and harassment since beginning their work in June last year.
"It is really disheartening to hear of the harassment that our panelists have faced online because they're studying this topic," NASA's science chief, Nicola Fox, said in her opening remarks. "Harassment only leads to further stigmatization."
The greatest challenge panel members cited, however, was a dearth of scientifically reliable methods for documenting UFOs, typically sightings of what appear as objects moving in ways that defy the bounds of known technologies and laws of nature.
The underlying problem, they said, is that the phenomena in question are generally being detected and recorded with cameras, sensors and other equipment not designed or calibrated to accurately observe and measure such peculiarities.
"If I were to summarize in one line what I feel we've learned, it's we need high-quality data," Spergel said. "The current existing data and eyewitness reports alone are insufficient to provide conclusive evidence about the nature and origin of every UAP event."
Taboos surrounding the issue also remain.
While the Pentagon in recent years has encouraged military aviators to document UAP events, many commercial pilots remain "very reluctant to report" them due to the lingering stigma surrounding such sightings, Spergel said.
The NASA advisory panel represents the first UFO inquiry ever conducted under the auspices of the U.S. space agency for a subject the government once consigned to the exclusive and secretive purview of military and national security officials.
