Pentagon officials say they cannot explain reports of a missile launch off the coast of California on Monday.
A CBS News helicopter captured what looked like the vapour trail of a missile rising from the water about 35 miles (56 km) offshore.
Some physicists said the trail was left by an aeroplane, and that on a clear day the vapour created an illusion making it appear to rise vertically.
The Pentagon does not consider the incident a threat.
"Right now all indications are that it was not [defence department] involvement in this launch" Pentagon spokesman Col David Lapan said.
"So far we've come up empty with any explanation. We're doing everything we can to try to figure out if anybody has any knowledge of what this event may have been."
Under normal circumstances, the launch of a US missile would require several different authorisations and notifications, but none are evident.
It is unclear if the suspected missile was launched from land or sea.
Physicist Michio Kaku of the City University of New York said a frame-by-frame analysis of the video revealed the trail was created by an aeroplane.
He told CNN on Wednesday the object leaving the trail was travelling at a constant velocity and deviating in its direction, while rockets accelerate as they leave the ground and move in a straight line.