A day after a Palestinian flag was lowered from the Manhattan Bridge -- the second flag-related security breach on the East River in as many months -- police still don't know who to blame.

The flag, which appeared draped from the lower level Wednesday evening, was quickly taken down by police. Nearby, a pro-Palestinian march made its way across the Brooklyn Bridge, a witness said.

On the flag the words "Boycott," "Divest," "Sanction," and "Gaza in our hearts" were written.

On Thursday the Anti-Defamation League denounced both the march and the lowering of the flag.

"We strongly condemn this effort to delegitimize Israel which was on full display on the Manhattan Bridge," said Evan Bernstein, the group's New York regional director, in a statement. "Individuals have every right to express their views, however there is no place for this type of display in New York City or in any city across the country."

Last month, the American flags atop the Brooklyn Bridge were replaced with white flags, drawing the ire of several politicians, including Mayor Bill de Blasio. A pair of German artists claimed responsibility for the stunt and said they did it to honor the bridge's German engineer, John Roebling, who died on July 22, in 1869.