Transcript for American Flags Missing From the Brooklyn Bridge
And now, here in New York, a mystery atop one of the most famous landmarks in the nation. And it is raising real questions about security here. The Brooklyn bridge, and the two American flags crowning the majestic spires, but today, in their place, two white flags. So, who put them there and why? ABC's linsey Davis with clues tonight. Reporter: An eerie sight this morning -- flying high above the brroklyn bridge -- two white flags in place of the American flags typically hoisted above these iconic towers. The ability to pull off a stunt on this scale is as troubling as it is mysterious. There is some indication of some good deal of preop rational planning. Reporter: The bridge sweeps over New York's east river, spanning more than a mile, the towers standing 276 feet tall. The cables holding it all together, 16 inches in diameter, each comprised of more than 5,000 galvanized steel wires. The stars and stripes normally fly around the clock, but take a look at this Earth cam footage. At 3:30 this morning, the waving American flag suddenly disappears. Watch again. That light seems to go out. Police today saying these aluminum plans were used to cover the bridge lights. Those white flags -- actually old glory bleached white. Today, as workers scaled the bridge, taking down these symbols of surrender, lice reviewed surveillance tape showing four to five people crossing the bridge about 20 minutes before the flags came down. They're telling you that your security is not effective. What more of a tell-tale sign do you need to show you that you're not doing a good job? Reporter: This landmark considered such a high profile terrorist target, it's under 24 hour police surveillance. Just in this area alone at the entrance of the bridge, in the span of about 20 yards, count them, one, two, three security cameras and then a sign alerting you to their presence. Tonight, the NYPD promising a full probe, even as specially trained workers hoisted American flags once again. Those American flags back up tonight. And police saying they believe whoever was responsible likely had some kind of training in climbing or construction. But they do not believe that this was a terrorist act. They're saying this was even possibly an elaborate art project.
This transcript has been automatically generated and may not be 100% accurate.