Speaking by telephone from Germany, the artists cited Gordon Matta-Clark, who famously climbed the Clocktower Building in Lower Manhattan in 1974, among other American artists whose uncommissioned actions in and around the streets of New York inspired them.

“We saw the bridge, which was designed by a German, trained in Berlin, who came to America because it was the place to fulfill his dreams, as the most beautiful expression of a great public space,” Mr. Leinkauf said. “That beauty was what we were trying to capture.”

They volunteered that the flag project transpired roughly between 3 and 5 a.m. on July 22. They said they carried the white flags in backpacks up the climbing cables that workers and the police use to reach the towers, and did not see security cameras. They would not say whether other people were involved.

They had made the flags themselves, they said, spending more than a week hand-sewing them with two kinds of white fabric, alternating the fabrics to make stripes, cutting out holes for the stars from one fabric and filling them in with the other. At about 10 by 19 feet, the white flags approximated the size of the American flags on the bridge.

The artists stressed that when they removed those flags, they ceremonially folded them, “following the United States flag code,” Mr. Leinkauf said. The flags will be returned, he promised. As with their other projects, he stressed, the plan was always to come forward. “We always face the consequences,” he said. “This is part of the work, to have an open discussion. We just needed a little time to decide how to respond to the reaction.” They are now considering legal advice.

Their website suggests that for roughly a decade, Mr. Leinkauf, 37, and Mr. Wermke, 35, have focused on the often illegal margins of public spaces. “Places people pass through or pass by, but don’t usually notice, like tunnels and bridges or the tops of buildings,” Mr. Leinkauf elaborated.