Dr Vivek Shah, who works at New England Baptist Hospital in Boston, was about to cross the finish line when he was put to the ultimate test.
Dr Shah and his running mates heard a loud boom, but weren't sure what had happened. So they continued toward the finish line. Then came the second blast.
"All the runners and spectators started running towards us, away from the finish line. But my entire family, my wife, my daughter, my parents and my sister were all at the finish line. So I started running toward where I heard the boom," Dr Shah was quoted as saying by the WCVB.com.
Concerned for his family, he had no idea what he was about to witness.
"People with traumatic amputations, one leg, both legs, it just looked like everyone was in shock. If you look into the victims' eyes, they didn't really know where they were. I've never seen that quantity of injury in one place," said Dr Shah.
"I just tried to see if anyone needed any emergent care, if anyone was bleeding out. We put on some makeshift tourniquets."
But within a minute, he said, emergency physicians were on site, attending to every victim.
"As soon as the area was stable with lots of personnel, I tried to find my family, because my biggest concern was that one of the faces that I'd see would be theirs," Dr Shah said.
Thankfully, his family was fine.
"They didn't leave the area even though they were being told to evacuate. They were just hoping that I would show up.
They were only 25 yards from where the last blast was," Dr Shah said.