It's a mystery that has many puzzled.
The four people captured on video entering Brooklyn Bridge around the time two bleached-white American flags were planted there appear to be in their early 20's.
One was carrying what appears to be a skateboard, a senior law enforcement official tells NBC News.
The flags were found fluttering early Tuesday from poles perched on the stone supports where two American flags are normally positioned.
The official said the NYPD has video showing four people entering a walkway on the bridge at about 3:30am, close to when the lights that normally illuminate the American flags on top of the bridge went out.
One of them had what "possibly looks like a skateboard."
Police don't yet know who the young people are, and are continuing to scour surveillance video and social media in their investigation.
Emergency service unit officers of the NYPD retrieved the bleached American flags before noon Tuesday.
They also found large aluminum pans affixed over the flag lights, secured with zip ties.
The high-quality flags, measuring about 11x20 feet, were made out of white linen, and the stars were individually stitched on, according to the senior law enforcement official.
The NYPD is looking at forensic evidence on the tin pans as well as on the flags.
Because the gate to the top of the bridge was found still locked, Deputy Commissioner, John Miller said Tuesday there was "some indication of pre-operational planning," perhaps by someone with experience climbing in construction or in bridge work.
There's no indication that the act was a statement of terrorism or politics, Miller said.
The American 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're lit from the bottom by a lamp at the base of each tower at night.
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 the crossing.
The bridge is said to be one of the most heavily secured landmarks in the city, constantly monitored by surveillance cameras.
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