Video of the first bomb going off was caught on a Boston Marathoner's own helmet cam.
Jennifer Treacy is the runner who's video is now being seen around the world.
"For the first split-second I thought it might be fireworks, but then I realized it was probably a bomb because it was so loud," she said.
Soon after the horror of Marathon Monday, her daughter posted the video on YouTube with the message "My mom didn't finish the race, but we are all safe and that's what matters. My heart goes out to victims." It has been viewed more than one million times—and counting.
She said, "I thought, 'This is my first Boston Marathon.' I really wanted to savor the whole experience and remember it."
On Wednesday, the FBI came calling and asking for Treacy's video. Investigators took it all, and on mile 22, the video showed Treacy greeting her family members cheering her on from the sideline.
Little did she know that as she approached the finish line, her camera would capture a moment that will go down in history: the first bomb detonating. The video showed Treacy's feet turning around as she tried to get away.
"Everyone was panicking, trying to find their loved ones, and it was horrible," she said.