The following recounts an incident in 2004 that advocates of research into U.F.O.s have said is the kind of event worthy of more investigation, and that was studied by a Pentagon program that investigated U.F.O.s. Experts caution that earthly explanations often exist for such incidents, and that not knowing the explanation does not mean that the event has interstellar origins.

Cmdr. David Fravor and Lt. Cmdr. Jim Slaight were on a routine training mission 100 miles out into the Pacific when the radio in each of their F/A-18F Super Hornets crackled: An operations officer aboard the U.S.S. Princeton, a Navy cruiser, wanted to know if they were carrying weapons.

“Two CATM-9s,” Commander Fravor replied, referring to dummy missiles that could not be fired. He had not been expecting any hostile exchanges off the coast of San Diego that November afternoon in 2004.

Commander Fravor, in a recent interview with The New York Times, recalled what happened next. Some of it is captured in a video made public by officials with a Pentagon program that investigated U.F.O.s.