Patriots Day in Boston is normally a celebration. The hotels are full, the bars are packed and people often take in the Boston Marathon before heading out to have fun.
I am standing now in what is normally a busy street near the heart of the city, but there's hardly anyone here.
At the end of the street, I can see the flashing lights of a dozen or more emergency vehicles. They have nowhere to go but they sit with engines running.
Six blocks away, police specialists and FBI experts are combing through the wreckage of the twin bombings which hit the end of the famous marathon.
The video is clear, loud and graphic. The crowds cheer and the runners run, summoning the last ounce of energy to carry them across the finishing line. When suddenly, there is a loud thump.
The screams of delight turn to those of fear and terror. A plume of smoke rises into the air as people on the side of the road lurch forward. And before anyone can catch their breath, just a hundred metres away, a second explosion. Louder. Darker. Somehow much more terrifying.
As people rush to help those in need, it is clear there are many injuries. Blood soaks the pavement. Limbs and lives have been lost.
The blasts were not huge, but nails and ballbearings were wrapped around the bombs, cynically and ruthlessly making them much more effective at killing and maiming.
Now comes the investigation. And the inevitable question of who was behind this.
There are those who suspect it may have been a domestic attack. They point to the day – 'Patriots' Day' that other attacks such as Columbine, Waco and the Oklahoma City bombing all happened in this same week, several years apart. They noticed there was no "chatter" ahead of the event that it may be targeted which discounts the usual suspects.
But the police have identified a 'black or dark skinned man with a foreign accent' that may be a person of interest. The double bombings – with other devices discovered – suggest similarities with attacks seen in Iraq and Afghanistan.
The inquiries into this tragedy will call on every resource they need. And it will be a fast moving inquiry, but it could still take days, even weeks.
When Boston wakes on Tuesday, it will be a different place. There will be more security. There will be more checks and precautions. There will be a wariness of the unusual and the different.
And people will always remember the time when Patriots' Day stopped being a celebration.