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There was a boom; then a hum. The lights flickered. A giant plume of smoke filled the New York City sky around 9:12 p.m., and turned it blue.

“A sort of unnatural, fluorescent shade of blue,” said Bill San Antonio, 28, who was watching Thursday night from inside a terminal at La Guardia Airport.

“We thought it was a U.F.O.,” said Yiota Androtsakis, a longtime Astoria resident.

Ms. Androtsakis was not the only one. In the earliest moments, hundreds of Twitter users from across the city posted videos of the eerie lights, causing many on social media to fear an alien invasion.