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For hours on Monday, fog surrounded the skyscrapers in Midtown Manhattan, hiding the upper floors behind a gauzy, grayish curtain.

About the time that the late-lunch crowd would have been signaling for the check, there were noises that seemed out of place, even in noisy New York City: the disturbing roar of an aircraft flying low, followed by what some assumed was an explosion.

A helicopter had crashed onto the roof of an office building on Seventh Avenue and burst into flames.

Only a pilot was aboard the doomed aircraft. He was killed, and investigators were trying to determine if he had been trying to make an emergency landing.

Alerts spread on cellphones as a smoky plume streamed through the fog. New Yorkers, unnerved, wondered whether the crash had been deliberate. It rekindled memories of a far different day — Sept. 11, 2001, when jetliners commandeered by terrorists destroyed the twin towers of the World Trade Center.