Rescue workers at the scene of Monday's bombings went into automatic overdrive - springing into action to save lives. Within 18 minutes of the blasts, EMTs cleared the streets of all critical victims, risking their own lives.
Now, in the aftermath, these heroes are grappling with the horror of what they saw.
Poppy Harlow is Out Front tonight with their stories.
As investigators search for a break in the case, there is new evidence that may be the best lead yet. CNN and WAGA in Atlanta have obtained new images of the explosive device that killed three people and wounded more than 180 others.
But is this new evidence - the lead they have been looking for?
Outfront tonight: Don Borelli, a former member of the FBI's joint terrorism task force.
It's been more than 32 hours now since two explosions went off at the finish line of the Boston Marathon.
It's a crucial time period for investigators as they search for a suspect.
OutFront tonight: Andrew Gumble, author of Oklahoma City Bombing: What The Investigation Missed – and Why It Still Matters and Sean Smith is a former assistant secretary of Homeland Security.
Congressman Stephen Lynch represents the neighborhood of Dorchester section of Boston, the home of the Richard family of 8-year-old Martin Richard, who was killed in the blast. His 6-year-old sister had one leg amputated, and may lose the other. And his mother suffered a brain injury.
Erin Burnett talks to Rep. Lynch, a long-time friend of the Richard family about the loss of their 8-year-old son.