Police say they're looking for a group of four to five people in connection with the mysterious appearance of two white flags atop the Brooklyn Bridge.
They were found fluttering early Tuesday from poles perched on the stone supports where two American flags are normally positioned. Police removed both white flags before noon.
Deputy Commissioner John Miller said in a news conference that security video shows a group of people crossing the bridge overnight. Police want to talk to them as part of their investigation.
"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."
Miller said the lights that normally illuminate the American flags on top of the bridge flickered and then went out at about 3:30 a.m., first on the Brooklyn-side tower, then on the Manhattan-side tower, video shows.
At about 5:30 a.m., construction workers noticed the American flags had been replaced by the white flags and reported it to authorities. Emergency service unit members of the NYPD went to the top of the bridge and found the flags, which appeared to be American flags measuring 20 feet by 11 feet that had been bleached white.
The ESU officers also found large aluminum pans affixed over the flag lights, secured with zip ties. A city employee estimated it could take up to two hours to take a flag down from the bridge and raise another up.
Because the gate to the top of the bridge was found still locked, MIller said 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. Police are looking across social media to see if anyone is trying to claim responsibility, and are asking for the public's help in their investigation.
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 are 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 one of the most heavily secured landmarks in the city, constantly monitored by surveillance cameras.
Brooklyn Borough President Eric Adams said in a statement that if someone swapped the flags as a joke, he didn't find it funny.
"The public safety of our city is of paramount importance, particularly our landmarks and bridges that are already known to be high-risk targets," Adams said. "We will not surrender our public safety to anyone, at any time."
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 the tower.
-- Marc Santia and Brynn Gingras contributed to this report.
Published at 10:32 AM EDT on Jul 22, 2014 | Updated at 12:43 PM EDT on Jul 23, 2014
Copyright Associated Press / NBC New York