The switch took place Tuesday atop the bridge's two 276-foot towers. The U.S. flags that normally fly were replaced with two bleached white U.S. flags.
A police source said DNA evidence had been collected from one of the tin pans used to block out the lights on the towers when the switch took place.
Police said so far the DNA doesn't match any biological evidence already in the New York Police Department system.
The bridge, which opened in 1883, was designed by John A. Roebling and was one of the first suspension bridges in the United States.