Investigators are running license plate numbers, scouring social media, examining cellphone transmissions and collecting DNA as part of their investigation into who climbed the Brooklyn Bridge in the middle of the night to plant two bleached-white American flags there, NBC News has learned.

The flags were found fluttering early Tuesday from poles perched on the stone supports where two American flags are normally positioned. They were taken down by noon that day.

The high-quality flags, measuring about 11 feet about 20 feet, were made out of white linen, and the stars were individually stitched on, according to a senior law enforcement official. There were also large aluminum pans affixed over the bridge lights that normally illuminate the flags, secured with zip ties.

The NYPD has obtained a small amount of DNA from the flags or tin pans that investigators are now seeking to test, a senior NYPD official tells NBC News. They're also looking at cellphone transmissions from the bridge in the time frame that the flags were swapped.

Investigators are also in the middle of a painstaking process of elimination: the lights on the bridge went out at 3:27 a.m., and in the hour around that moment, about 1,800 cars were crossing the bridge. Police are running all of those license plates, the official said.

Earlier this week, NYPD brass said they were particularly interested in anyone with climbing experience or insider knowledge of the bridge. Detectives have already interviewed the construction workers who do repair work on the bridge each night, according to the official.

A company called Skanska Koch has a $500 million contract to refurbish the bridge. Ed Hartnett, a former NYPD intelligence commander and president of Brosnan Risk Consultants, a firm that advises contractors on how to secure their work zones, said that on a project like the Brooklyn Bridge, Skanska should have its own camera network and its own security lighting.

In addition to all the leads detectives are running down, they're still trying to identify a group of young men, one with a skateboard, who crossed the bridge around the time of the flag swap. They're still considered persons of interest, but it's not clear if it's because they were involved or just may have been witnesses.

There's no indication that the act was a statement of terrorism or politics, officials have said.

The bridge is one of the most heavily secured landmarks in the city, constantly monitored by surveillance cameras.



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.