New York Mayor Bill de Blasio says a security breach involving the changing of flags at the Brooklyn Bridge last week is "a wake-up call."
De Blasio said Monday police have increased security on all the city's bridges.
Police believe four or five people climbed the bridge's two towers on July 22 and replaced the giant American flags with white ones.
The operation involved disabling a pair of spotlights and appears carefully planned. The culprits remain at large, and their motives are unknown.
De Blasio was in Italy during the security failure at one of the city's most famous landmarks. He says in his first comments about it he's "absolutely concerned."
Some people fear a group with more sinister motives could penetrate the bridge's security.