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
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