Video posted on the CBS News website shows an object flying through the evening sky Monday that left a large contrail, or vapor trail. A news helicopter owned by KCBS, a CBS affiliate in Los Angeles, shot the video.
Pentagon officials were stumped by the event. "Nobody within the Department of Defense that we've reached out to has been able to explain what this contrail is, where it came from," Pentagon spokesman Col. Dave Lapan said.
While the vapor cloud captured on video resembled that created by a rocket in flight, military officials said they didn't know of any launches in the area.
One expert called it an optical illusion. "It's an airplane that is heading toward the camera and the contrail is illuminated by the setting sun," said John Pike, director of the U.S.-based security analyst group globalsecurity.org.
The North American Aerospace Defense Command, or NORAD, issued a statement jointly with the U.S. Northern Command, or NORTHCOM, saying no Department of Defense entities reported a missile launch - scheduled or inadvertent - at the time of the contrail, and no foreign military missile launch was detected.
NORAD said it determined there was no threat to the U.S. homeland.
The Federal Aviation Administration ran radar replays from Monday afternoon and they "did not reveal any fast-moving, unidentified targets," the statement said. No pilots reported unusual sightings to the FAA.
NORTHCOM is the U.S. defense command and NORAD is a U.S.-Canadian organization charged with protecting the U.S. from the threat of missiles or hostile aircraft.
Pike said the object could not have been a rocket because it appeared to alter its course.
"The local station chopped up the video and so it's hard to watch it continuously," Pike said. "But at one place you can see it has changed course - rockets don't do that."
Pike said he didn't understand why the military had not recognized the contrail of an aircraft. "The Air Force must ... understand how contrails are formed," he said. "Why they can't get some major out to belabor the obvious, I don't know."
Associated Press writers Jeff Wilson and John Antczak in Los Angeles contributed to this report.