It's one of the most heavily-guarded landmarks in all of New York City yet somehow someone snuck onto the Brooklyn Bridge and replaced two American flags with all white versions.
"We don't take these things lightly, or as a joke, or as art."
That's because they know how easily those perpetrators could have planted bombs on the bridge instead of flags.
It's the latest reminder of how vulnerable America still is to terrorist attacks.
Highlighted in this just-released 911 commission report laying out a growing range of diverse threats: from home-grown terrorists to overseas attacks on American computer systems.
"This is a new dangerous phase the United States of America is entering into."
The 9-11 commission warned, a decade ago, if Iraq falls, terrorists will fill the vacuum.
And as the commission's new report says, "That nightmare scenario may now be coming to pass."
The terrorist group ISIS is taking over sections of Iraq and Syria and ramping up recruiting efforts. Convincing Europeans and Americans to come train with them in the Middle East. US authorities worry they could come back to launch attacks
"People starting to come back from these training grounds to the United States, Al Qaeda now, pre-9/11 they were in a few countries, now they are in 16 countries around the world."
And Al Qaeda is still pushing to hurt Americans.
Its Yemeni arm recently laying out in its english-language terrorist magazine "Inspire" a list of the most coveted American targets, such as this Summer's U.S. Open tennis tournament in New York.
Counterterrorism officials are concerned this will serve to encourage lone-wolf terrorists flying under the radar.
One of the biggest threats right now didn't even register when 9-11 commission wrote it's original report. Computer attacks that could wipe out key parts of the infrastructure like the power grid or banks.
So as the threats grow -- on every front -- homeland security experts say the mission today is clear.
"To understand, to be proactive and to be smart about the changes taking place in the world before we are attacked again is one of the most important lessons in our report."
A lesson with a bold reminder right in the face of New York.