Mysterious white flags replace US stars and stripes on Brooklyn Bridge

New Yorkers awoke to find two white flags hung in place of the American flags that traditionally flutter atop the Brooklyn Bridge on Tuesday morning, baffling city officials, police and onlookers.

Two US flags usually decorate the towers of the iconic bridge, which opened in 1883, but on Tuesday morning on they had been replaced with stark white flags that, on closer inspection, appeared to be the stars and stripes with all the colour bleached out, police said.

The white flags have since been taken down, and the red, white and blue ones restored.

"This may be somebody's art project or may be somebody's attempt at making some kind of statement," John Miller, the deputy commissioner for intelligence and counter-terrorism, said at a press conference, "but it's not clear what that statement is."

Video camera footage shows several people, including some four or five people in a group, crossing the bridge at about 3:30am, Miller said.

A few minutes later, the light illuminating the flag on the bridge's Brooklyn tower flickers and then goes out.

Soon after, the sequence repeats atop the Manhattan tower.

Construction workers arriving a couple of hours later discovered the minimalist flags, which appeared to be commercially made and measured about 20 feet by 11 feet (6 meters by 3 meters).

Investigators also found aluminium pans that had been affixed over the flags' lights, Miller said, suggesting some careful planning had gone into the effort.

Police are searching for the people behind the switch and no arrests have been made.

The towers' security gates were still locked, suggesting the perpetrators were particularly agile and may have construction experience, Miller said.

"We don't take these things lightly, or as a joke, or as art, or within the realm of speech," Miller said, adding that the police considered the flag-placing an act of trespass.

The bridge is seen as a prime target for acts of terrorism, and is closely monitored by police.

Miller said the department is looking into whether it needs to increase the number of patrols and cameras on the bridge.

In April, police arrested Brendan Fagan, a street artist who goes by the alias of Judith Supine, after he clambered up the Queensboro Bridge further up the East River at night and installed an artwork.