LOS ANGELES >> Gil Leyvas has been a photojournalist on board a television news helicopter for more than a decade. He has seen countless airplanes and their wispy contrails. What he saw — and recorded — near Los Angeles on Nov. 4 and 8 looked nothing like the trail from an airplane. It looked, to him, like the launching of a missile.

The first time, it looked like a far-off plume of smoke somewhere over the Pacific Ocean. The second time, it appeared to be rising into the air, a large vertical column set against the bright orange sky at sunset. It was spectacular.

The sunset video piqued the attention of KCBS, the TV station Leyvas works for, and by dawn the next day, last Tuesday, news anchors were speaking of a “mystery missile,” one that apparently posed no danger to Los Angeles but that baffled the people who saw it. By the end of the day, the video had garnered worldwide attention, which the absence of an official government explanation only magnified.

All the authorities could say at first was that there was no radar evidence of any craft in the area. So people put forward all manner of theories: It was a classified Navy test. It was a provocation by a foreign power. It was a publicity stunt for a television show about aliens.

On Wednesday, about 30 hours after the “mystery missile” started attracting news media attention, a Pentagon spokesman said that “there is no evidence to suggest that this is anything else other than a condensation trail from an aircraft.” That same day, news coverage took a sharp turn, with many reporters, and experts, concluding that what Leyvas had seen was an airplane or, barring that, an optical illusion. Some experts chastised media outlets for running with a half-baked, whole-hyped story.

That has not stopped the guesswork, however. And it has not stopped Leyvas from wondering what caused the contrails. “I’m trained to look for something that’s out of the ordinary, and this was out of the ordinary,” he said in an interview on Thursday.

Leyvas said that he had never seen an airplane contrail that resembled the contrails on Nov. 4 and Nov. 8. In fact, while he was recording the contrail on Nov. 8, he briefly panned the camera away when he saw a second contrail in the distance, only to discover that the second one had been caused by a jetliner. “There was no comparison at all” between the two, he said, because the mysterious one was several times bigger.

He added, “The video speaks for itself.”

Experts have clashed in recent days on whether the mystery plume came from a missile or an aircraft, and they now tend to favor the aircraft explanation. Many, however, say that the available evidence cannot rule out a missile.

“I’m 98-percent sure it was a contrail,” or engine vapor from an airplane, said Jonathan McDowell, a Harvard astronomer who tracks rocket launchings and space activity. But he said that scores of missiles flew off launching pads in Southern California every year — and that a number of them were highly classified tests of foreign missiles that American intelligence agencies had managed to acquire.

A main site of secretive work, McDowell said, is San Nicolas Island — a speck in the ocean about 75 miles west of Los Angeles that the military has used for decades. Last week, some experts suggested that San Nicolas seemed to lie in the vicinity of the mystery plume.

Theodore A. Postol, a physicist and former Pentagon science adviser who teaches at the Massachusetts Institute of Technology, said that he, too, had come around to the jet explanation, but he also urged interpretative caution.

“I can’t rule out a missile launch,” he said. “The argument that it’s a contrail from an airplane could be wrong, but it’s not dismissible.”

Postol said he was amazed that the federal and military authorities let the episode fester for days with a series of inconclusive answers.

“These people are responsible for the air defense of the United States,” he said in an interview. “Yet they did a lot of hand waving and left the impression that they were not telling the whole story. That’s not responsible for the people with their kind of responsibilities.”

The Pentagon said that no Department of Defense entities had reported any scheduled or inadvertent launchings of rockets or missiles on Nov. 8. When KCBS inquired about the contrail on Nov. 4, it found that there had been no launchings scheduled that day at Vandenberg Air Force Base, north of Los Angeles, where satellites are launched into orbit.

But some people, including John E. Pike, director of GlobalSecurity.org, said from the start that the tape showed an airplane. In an interview, Pike, whose group in Alexandria, Va., analyzes space and military technologies, defended the military’s evasiveness as resulting not from dissembling but from the difficulty of knowing with certainty what every part of its vast network was up to.

“I think it temporarily confused the Pentagon,” he said of the plume. “They had to triple-check to see if they actually did have something going on out there, to see if there was some black program they should not talk about.” In military jargon, black is a synonym for secret.

Pike added that television news programs had acted irresponsibly in pushing the missile thesis without bothering to establish basic facts that would have quickly cleared up the riddle.

“To me, it demonstrated the extent to which news organizations are captives of their sources,” he said.

McDowell of Harvard drew another lesson. “It’s a great teaching moment,” he said. “Just because there’s an obvious interpretation doesn’t mean there isn’t something more subtle going on. It’s always worth taking a second look to see if there’s an alternative hypothesis that might fit the data.”

Diener, the KCBS news director, said that the experts interviewed by KCBS on Tuesday night and Wednesday morning had leaned toward the missile theory, spurring the initial “mystery missile” coverage. Asked why he thought there had been a change in the tone of the coverage, he surmised that the aircraft was “the explainable answer as opposed to the unexplainable.”

© 2010 The New York Times Company