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Experts believe the mystery missile fired off the Californian coast may have been an optical illusion.
The unknown missile, which was caught on camera by a news helicopter on Monday has baffled many people in America.
But US-based security analyst, Jim Pike, thinks he has solved the mystery.
He told CBS News the object could not be a rocket because it appears to alter its course and concluded it must be an optical illusion.
Mr Pike said he believes the video shows an aeroplane heading towards the camera with the plane's contrail illuminated by the setting sun.
The launch took place around 5pm local time on Monday and the location was described as west of Los Angeles, north of Catalina Island and about 35 miles out to sea.
Pentagon spokesman Colonel Dave Lapan said all indications are that the defence department was not involved and the missile's trail might have been created by an object flown by a private company.
It is not known who fired the missile
"Nobody within the Department of Defence that we've reached out to has been able to explain what this trail is, where it came from," he said.
Col Lapan added: "At this point the operative term is unexplained."
The North American Aerospace Defence Command (Norad) and the US Northern Command (Northcom) said the trail was not the result of a foreign military launching a missile.
They said: "We can confirm that there is no threat to our nation."
A US Navy spokesperson said it was not their missile, adding there was no navy activity reported in that part of the region.
Missile tests are common off southern California. Launches are conducted from vessels and platforms on an ocean range west of Point Mugu.
Military officials are examining the possibility it was a "commercial launch of some kind", or that amateurs had built a device capable of creating such a plume.
The missile was caught on camera by a news helicopter
Last Friday, Vandenberg Air Force Base in California launched a Delta II rocket, carrying the Thales Alenia Space-Italia Cosmo SkyMed satellite.
But a sergeant at the base said there have been no launches since then.
Robert Ellsworth, former US ambassador to Nato and an ex-deputy secretary of defence, said: "It's spectacular… It takes people's breath away."
He called the projectile "a big missile" but said it did not appear to be a Tomahawk.
Mr Ellsworth said it might have been a missile test timed as a demonstration of American military might as President Obama tours Asia.
"It could be a test-firing of an intercontinental ballistic missile (ICBM) from a submarine… to demonstrate, mainly to Asia, that we can do that," he speculated.
Mr Ellsworth said such tests were carried out in the Atlantic to demonstrate America's power to the Soviets, when there was a Soviet Union.
But he does not believe an ICBM has previously been tested by the US over the Pacific.
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