An object shooting across the sky and leaving a large vapor trail over the Pacific Ocean near California (Photo: AP).
The Pentagon is trying to find out if a missile was fired off the California coast, and who launched it.
What appeared to be a large missile was seen shooting into the air about 35 miles west of Los Angeles and leaving a long vapour trail, or contrail, over the Pacific Ocean.
The US Navy, Air Force, and other military organisations are examining video of the event captured from a passing news helicopter.
Colonel Dave Lapan, a Pentagon spokesman, said: "Nobody within the Department of Defence that we've reached out to has been able to explain what this contrail is, where it came from.
"Right now all indications are that there was not Department of Defence involvement in this."
There was speculation that the trail could have been caused by a projectile launched by a private company.
However, any private missile test would have required notification so that boats and planes in the area could be warned, or busy air space near Los Angeles International Airport closed.
Missile tests are common off southern California, with launches being conducted from vessels and platforms out in the Pacific.
The North American Aerospace Defence Command (NORAD) and the US Northern Command (NORTHCOM) issued a joint statement saying that the vapour trail was not the result of any foreign military launching a missile, but gave no further details.
Robert Ellsworth, a former US Deputy Secretary of Defence, told KFMB, a CBS affiliate in San Diego, one theory might be that it was a military muscle-flexing ploy.
"It could be a test firing of an intercontinental ballistic missile from an underwater submarine, to demonstrate mainly to Asia, that we can do that", he said.