BROOKLYN BRIDGE -- Police are searching for four or five people they believe scaled to the top of the Brooklyn Bridge's two towers in the dead of night, disabled lights illuminating two large American flags and then replaced the flags with bleached-white ones.The security breach at one of the city's most secured landmarks didn't appear to be the work of terrorists or even a political statement, said the police department's deputy commissioner for counterterrorism and intelligence, but was likely done by people familiar with climbing or bridgework who may even have scaled the bridge before."We don't take these things lightly, or as a joke, or as art or within the realm of speech," John Miller said. "These are issues of trespass - they put themselves in danger, they put others in danger - and that's why we investigate it."Video footage of the security breach shows the unidentified people walking on the bridge's footpath at about 3:10 a.m., and 20 minutes later the light on the bridge's Brooklyn tower flickers and goes dark, Miller said. The same thing happens about 12 minutes later on the Manhattan tower, he said.Locked gates midway up the main cables leading to the tops of the towers didn't appear to have been tampered with, suggesting the climbers scaled them to reach the top, Miller said.Two police cars sit at either end of the bridge, which stretches the East River connecting lower Manhattan and Brooklyn, and are fed real-time security camera footage trained on areas affecting the structural integrity of the bridge, Miller said, but those cameras didn't capture the flag bandits.At about 5:30 a.m., construction workers noticed the white flags, apparently American flags about 20 feet by 11 feet and perhaps commercial grade, with faded stars and stripes, police said. Police removed the white flags just before noon.The flags fly from above the pillars year-round and are replaced by transportation workers when they become frayed about every two months, police said. They are lit from the bottom by a lamp at the base of each tower at night - lights that were covered by aluminum foil cooking sheets secured with zip ties, Miller said.More than 120,000 vehicles, 4,000 pedestrians and 3,100 bicyclists cross the Brooklyn Bridge every day, said the city's Department of Transportation, which maintains it.Tourist Johan Lund, from Stockholm, Sweden, crossed the bridge Tuesday and did a double take when he noticed the white flags flapping in the wind."Wasn't there an American flag there yesterday?" he said to himself.High-profile breaches have been made before.In April, a street artist who filmed his effort scaling the Ed Koch-Queensboro Bridge as part of an art installation was arrested on criminal trespass and other charges. And in March, four skydiving enthusiasts were charged with reckless endangerment for sneaking into 1 World Trade Center, the nation's tallest building, months earlier and filming their jump from atop it.