WASHINGTON — The Pentagon said Wednesday it is satisfied an aircraft -- and not a missile -- was the source of a vapor trail off Los Angeles this week that sparked fears of a mystery missile launch.

It took the Pentagon nearly two days to reach that conclusion after examining video of the plume, FAA radar tracks, its own missile launch detection systems and canvassing the US government.

"There is no evidence to suggest it was anything other than an airplane," said Colonel David Lapan, a Pentagon spokesman.

The vapor trail was videotaped by a KCBS News helicopter as it arced into the sky off Los Angeles on Monday evening, giving the appearance of a projectile rapidly moving through the air.

Speculation that it may have been a missile launch intensified on Tuesday after the Pentagon said it was unable to explain the source of the vapor trail.

Among the new evidence cited Wednesday by the Pentagon to conclude that it was an aircraft -- not a missile -- vapor trail are FAA radar images of the area, which detected several aircraft but no rockets, Lapan said.

"The FAA, when they looked at the radar replay for that time period, they went out some distance from the coast, and identified that they had no fast moving unidentified objects in that area at that time.

"They did have commercial airliners in that area. None of those commercial airliners reported anything unusual," he said. "So all of those things point again to the fact that we believe this is an aircraft condensation trail."

Until now, the Pentagon had been reluctant to confirm what outside experts were saying: that what the video had recorded was a common optical illusion.

Experts said that the vapor trail, or contrail, of an aircraft approaching the observer often looks like it is rising vertically into sky because the end of the tail is hidden by the curvature of the Earth.

"It's a jet plane contrail, obviously," said John Pike, director of GlobalSecurity.org.

He said no sign of a fiery exhaust was evident in the video, and that its movement was too slow and made changes of direction that were uncharacteristic of a missile trajectory.

But he faulted the Pentagon's halting, unforthcoming public response to the sighting for magnifying perceptions that either it was inept or it was hiding something.

"That's the mystery -- why I'm spending 750 billion dollars a year and that's the best I can get out of them?"

On Tuesday, the North American Aerospace Command, which is responsible for protecting the skies over the United States and Canada, issued a statement saying there was no evidence of a launch of a foreign military missile.

The command also gave assurances that there was no threat to US homeland.

But the military's initial response left open other possibilities, like a US military or civilian launch of some kind.

In Moscow, Major General Alexander Vladimirov, vice president of Russia's board of military experts, told the Interfax news agency the television image "looks like the launch of a missile from a submarine."

"Most likely we are talking about the launch of a Trident-2 ballistic missile from an Ohio submarine," Vladimirov was quoted as saying.

"There is reason to believe this was an unsanctioned launch of a missile from a submarine. If this is so, then many questions arise about the condition of the US armed forces," he said.

Lapan denied that the Pentagon's fumbling response to the vapor trail sighting was because of an attempt to cover for a secret, or "black" operation.

"That was one of the questions we asked yesterday," he said.

"That didn't take any more time than anything else. The time was canvassing all these different agencies for whatever information they might have, making sure everybody was certain this was something they didn't know about."