"There is no evidence to suggest that this is anything else other than a condensation trail from an aircraft," said Pentagon spokesman Colonel Dave Lapan, who reiterated that there was no threat to America.
The video of what looks for all the world like the contrail of a missile was shot Monday evening by KCBS cameraman Gil Leyvas from a news helicopter over Los Angeles.
"I saw a big plume coming up, rising from looked like beyond the horizon and it continued to grow," Leyvas said.
Meanwhile, an FAA official told CBS News: "We don't know the exact location, trajectory or altitude of the contrail, so we can't determine if it belonged to a specific aircraft."
Civilian experts agreed that the trail was left by an airplane, the Washington Post reports.
"This thing is so obviously an airplane contrail, and yet apparently all the king's horses and all the king's men can't find someone to stand up and say it," said John Pike, a defense and aerospace expert who runs GlobalSecurity.org.
The object, Pike told the Post, was moving too slowly to be a missile, adding: "There's a reason that they're called rockets."
He said the vapor trail was an optical illusion: The contrail appeared to start on the ground and but in reality, he said, the contrail began on the horizon and ran parallel to the ground.
Because it was "an unusually clear day," he told the Post that what looked like a missile launch 35 miles off the coast was actually the contrail of a jet that stretched 300 miles into the distance.
"At the end of the day, you really have to go with the simplest explanation," he added.
The Post reports that ContrailScience pointed to several similar incidents in the past in which jets were mistaken for missiles.