You hear the race announcer calling out the names of finishers one second. The next - clearly audible - comes the blast, ripping through one side of the course, dropping a runner to the ground, killing at least three people, maiming so many more.
'We've had an attack,' someone can be heard saying in the recorded footage. 'Oh my God! Oh my God! Oh my God!'
Of all places to ruin and end lives, literally yards from where so many of the more than 24,000 runners worked so hard to embrace a pure and noble goal - to run, and finish, a race. That's why so many of us started running, putting one foot monotonously in front of the other, moving forward. Of all places to attack the majesty of the human spirit: at the finish line.
The terrifying irony of the bloodshed Monday at the Boston Marathon: Almost nowhere can the epitome of endurance and resilience in a physical endeavor be found more than in the people who train and manage to run the race of the ancient Greeks - 26.2 miles.
Beyond the sponsored elite and the runners for charity, the Boston Marathon is still the crown jewel for weekend endurance warriors, a test of resolve that only the world's fastest qualifiers are allowed to run.
The most painful thing I ever witnessed in sports was a young woman nearly 100 yards from the finish line of the 2006 Boston Marathon. I had come to see my best friend try to break four hours - a long-held goal of his - and was suddenly caught up in the drama of this woman in a blond pigtail, no older than her mid-20s. Her legs began to wobble, her neuromuscular system shutting down stride by stride, until she began to walk and, finally, fell. She lay there in a fetal position, bawling inconsolably, in the middle of the course. Runners whizzed past. One finally stopped, trying to help her to her feet. But she couldn't make it.
A golf cart eventually pulled up and medical personnel attended to her and whisked her away, less than 100 meters from her dream. We didn't know her name, where she was from, what her story was. But bystanders wept because we knew the pain of not finishing what you started, coming so close to your goal and failing.
If you told me there would be bloodied limbs and panicked screams on the same street in almost the exact same area, I wouldn't have believed you - even in this post-9/11 world.
But there are many others within the running industry who worried a day like Monday could happen. A longtime race official I know who has worked five world-class marathons (Boston, New York, London, Berlin and Chicago), told me this had been many organizers' fear for years.
'The one opening in the security planning for everybody were the fans; there is no way to screen your fans,' said the official, who spoke on condition of anonymity. 'You can screen your runners, volunteers, media and race officials - you can make them all produce proof that they are who they say they are - but if the arena of competition is a city's streets, how do you possibly screen every person walking up to the race? You have to buy a ticket to get to the finish line area at many big races, but unfortunately anyone with money could procure one.'
Marathoners always talk about 'managing your pain,' about their innate ability to endure physical suffering more than so many other people they know. It's their rationale for so much torn cartilage and sore IT bands and plantar fasciitis and stress fractures.
From personal experience, having run four marathons, I know there are things worked out in your head on a run that cannot be worked out in a therapist's office or a friend's living room, a clarity only a physical journey can bestow. Putting in the miles, getting to a place of almost serenity and purpose in your running, can take you away from whatever was gnawing at you before you made the decision to put on your shoes and walk out the door.
There is also something about defiance in finishing, like the members of the New Orleans track club, who wanted to withdraw from New York in 2005 after Katrina ruined their city and lives. But stronger than the water was their will to be visible demonstrations of fighting back, the way so many others ran for someone they lost after 9/11.
For so many who ran, Boston on Monday afternoon was less about a race and more about a platform for so many courageous people to prove to themselves that life does go on, that closure is possible.
And to violently intrude on that kind of healing, that perseverance - to layer on tragedy and grief and heartbreak on the day so many watching and running were trying to move past it - is so wrong and personally destructive it's almost unspeakable.
No other major sporting event in the world can the novice line up next to the elite and try to spiritually climb their own Mount Everest. They all have a story of how they used their legs and their heart to conquer, overcome and get to a place of serenity or competitive satisfaction that finishing the oldest and greatest of marathons was supposed to bring them.
To hear, 'The race is over. There is no alternate finish,' announced at about 4:06 p.m., hurt so much deeper than pain of running 26.2 miles ever could.