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Two American flags that wave atop the two towers on the Brooklyn Bridge were mysteriously replaced with white flags sometime recently. New Yorkers began reporting the white flags on social media around sunrise Tuesday.

Police are investigating who may have placed them there, and how.

“We have several units there. We’re conducting an investigation, and it’s an ongoing and active investigation,” said Sgt. Lee Jones of the New York Police Department.

The white flags, which typically symbolize military surrender, were still flying over the iconic bridge at 11 a.m. Tuesday local time, but were taken down soon after. Live footage of the scene showed several uniformed individuals at the top of one of the towers, inspecting the base of the flagpole.

Jones said he didn’t know how an individual could have gotten to the top of the towers, or whether patrol cars typically guarded either side of the bridge.

The New York Department of Transportation referred all calls to the police department, which had no other information.

The Brooklyn Bridge is considered a high-profile location and has been under tight security since the terrorist attacks of Sept. 11, 2001. A man pleaded guilty in 2003 to plotting to destroy the Brooklyn Bridge by severing its suspension cables. He ultimately concluded that the plan would not work because security was too tight, court records said.