(CNN) -- New York police have collected DNA evidence in the case of nighttime acrobats who replaced two American flags atop of the Brooklyn Bridge with two white flags, according to a law enforcement official with knowledge of the investigation Friday.
The swapped-out bleached flags were spotted by construction workers at 5:30 a.m. Tuesday.
The DNA was taken from tin pans used by the vandals to cover bridge lights, according to the official -- a clever move to elude detection while the flags were raised. The tin shields were held in place with zip ties.
So far there is no match in existing databases where DNA evidence of solved and unsolved crimes are kept, the official added.
Among other leads investigators are following are five nicknames who tipsters suggest might be associated with the incident. Investigators are trying to learn the real identities behind the nicknames or street names. It's too soon to know whether any of them were involved, the official said.
The motive is unclear: Was it a stunt, prank or something more sinister?
Officials have said they are looking at whoever had access or familiarity with the bridge, including maintenance workers or anyone who worked on the fireworks extravaganza over the East River on Independence Day. The event this year included pyrotechnics shooting from the bridge.
Cell phone calls are being tracked and about 18,000 license tags were checked from vehicles that crossed the bridge in the overnight hours, according to the official.
More than 120,000 vehicles, 4,000 pedestrians and 3,100 bicyclists cross the Brooklyn Bridge every day, according to the city Department of Transportation, which maintains the bridge.
The oversized white banners -- American flags that were pre-bleached -- have prompted investigators to talk with flag manufacturers and check websites for anyone who may have recently purchased large Stars and Stripes, according to the official.
Intelligence analysts are also looking into any possible significance of the day that was chosen to see whether that may yield clues.
The NYPD's Intelligence Division has even reached out to other countries to see whether similar stunts have occurred elsewhere.
NYPD Deputy Commissioner for Intelligence John Miller said at a news conference Wednesday that the NYPD has video that shows four or five people crossing the bridge just after 3 a.m. Tuesday. Within the hour, the light that normally illuminates the flag on the Brooklyn side of the bridge flickered and appeared to go out. A few minutes later the same thing occurred on the tower on the Manhattan side.
"At this time, it appears it has no particular nexus to terrorism or even politics," Miller said Tuesday. "This may be somebody's art project, or it may be an attempt at making some kind of statement, but at this point it's not clear what the statement is."
Police Commissioner Bill Bratton called the incident a matter of concern and requested the help of the public as investigators search social media for claims of responsibility.
"If flying a white flag atop the Brooklyn Bridge is someone's idea of a joke, I'm not laughing," Brooklyn Borough President Eric Adams said in a statement. "The public safety of our city is a paramount of importance, particularly our landmarks and bridges that are already known to be high-risk targets."
Adams announced that he is offering a reward of $5,000 for information leading to the arrest of the person or people responsible.
The Brooklyn Bridge opened in 1883. At the time it was the longest suspension bridge in the nation, according to the Department of Transportation.
The bridge has been designated a National Historic Landmark by the National Park Service, and a New York City Landmark by the Landmarks Preservation Commission.
CNN's Poppy Harlow, Marina Carver and Rande Iaboni contributed to this report.