It was supposed to be a photo opportunity, a showcase of Air Force One alongside the sweep of New York City skyline. But as the low-flying Boeing 747 speeded in the shadows of skyscrapers, trailed by two fighter jets, the sight instead awakened barely dormant fears of a terrorist attack, causing a momentary panic that sent workers pouring out of buildings on both sides of the Hudson River. I thought there was some kind of an attack," said Paul Nadler, who sprinted down more than 20 flights of stairs after watching the plane from his office in Jersey City shortly after 10 a.m. "We ran like hell." Witnesses described the engine roar as the planes swooped by office towers close enough to rattle the windows and prompt evacuations at scores of buildings. Some sobbed as they made their way to the street. "As soon as someone saw how close it got to the buildings, people literally ran out," said Carlina Rivera, 25, who works at an educational services company on the 22nd floor of 1 Liberty Plaza, adjacent to the site of the Sept. 11, 2001, attack. "Probably about 80 percent of my office left within two minutes of seeing how close it got to our building." Neither the White House nor the F.A.A. explained why the mission was deemed a secret, even though officials conceded the primary purpose was picture taking. Officials at the Department of Transportation and at the Pentagon each denied responsibility for the secrecy.