In the chaos immediately after the Boston Marathon blasts, David Green took a picture with his smartphone before helping those who had been injured in the second blast.
It wasn't until three days later, when officials released video of the two suspects, that Green realized that he had a clear photo of suspect Dzhokhar Tsarnaev walking away from the scene.
Green, the CEO of a sportswear company who ran the marathon, immediately understood the importance of taking the picture. "I was in New York during 9/11," he tells PEOPLE. "You just have to document something, [so] I pulled my phone out and took one picture."
"Then I ran into the crowd to help whoever was there. I got back to the hotel, called the FBI and said I had the photo, then I posted it on Facebook."
"I called the FBI [again]," Green says. "Within 30 minutes, I spoke to half a dozen agents. I think they knew we had ... the best photo of suspect number two. So they told me I could circulate that photo. I re-posted it on Facebook, said 'This is a photo of suspect two, please get this out.' And between 7 and 11 p.m. on Thursday night, it went viral."
Now home in Jacksonville Beach, Florida, Green is thankful that he thought to take the photo. "It certainly can't make up for the tragedy," he says. "But it felt really good to be able to help."