Michael Winter

USA TODAY

The New York Police Department had a big whodunit on its hands Tuesday, in addition to egg on its face.

Who swapped out two big American flags flying over the Brooklyn Bridge towers with Old Glories that had been bleached white? And, more important, why?

But a more pressing concern was how did the overnight caper happen right under the noses of the NYPD on one of the city's most heavily guarded and constantly watched landmarks?

"We're lucky they just put a flag up there — and not a bomb," a law enforcement source told the New York Post. "It's an embarrassment."

Construction workers reported the swap at 5:30 a.m., NYPD anti-terrorism chief John Miller said at an afternoon news briefing. That was about 2½ hours after security video spotted five individuals crossing the bridge. Police are trying to identify the group.

The culprits climbed a locked gate and scaled a bridge cable to reach the top of the stone pillars.

Bridge video showed that at 3:30 a.m., lights atop the Brooklyn tower went out, followed 12 minutes later by lighting on its Manhattan twin. Police said the group used rectangular aluminum cooking pans fastened with zip ties to cover the lights.

The group replaced the 12-foot-by-18-foot American flags with two 11-by-20 Old Glories that had been bleached out, though the stars and the stripes could still be seen.

Crews began replacing the banners about 11 a.m. after police investigated and found no explosives or notes. Two new flags were up by 3 p.m.

Miller said the caper clearly involved "a good deal of preplanning."

The meaning of the white flags remained elusive late Tuesday, however.

"There's no particular nexus to terrorism or politics," Miller said. "It might be some kind of art project or statement. But we are not sure what that statement is."

Several news accounts pointed out that a white flag is the traditional sign of military surrender. A white flag is also a sign of a truce or cease-fire, raising the possibility this was a call for an end to fighting between the Israeli military and Hamas militants in the Gaza Strip.

Who pulled off the flag switch remains a mystery.

One group did take credit — and in the process briefly duped the New York Daily News and the Associated Press.

The "Bike Lobby" said on Twitter that it had raised the white flags "to signal our complete surrender of the Brooklyn Bridge bicycle path to pedestrians."

The parody account, which describes itself as an "all-powerful enterprise," was set up to make fun of bicycle opponents.

Fun ensued after the spoof cycled around the world.