Seconds after the Boston Marathon bombs exploded, David Green pulled out his smartphone and took a photo of the chaos developing a couple hundred yards in front of him -- the smoke, the people running in panic.
The Jacksonville businessman then put his phone back in his pocket and went to help the injured. It wasn't until Thursday, when officials released surveillance video of the two suspects in the twin blasts, that Green realized what he had -- a picture of 19-year-old Dzhokhar A. Tsarnaev, distinctive in his backward white baseball cap, walking away from the scene.
Tsarnaev, known to the FBI as Suspect No. 2, was still on the run Friday, prompting authorities to shut down the city of Boston and its western suburbs and warn residents to stay inside with their doors locked while police and federal agents hunted him down.
Tsarnaev's older brother, 26-year-old Tamerlan Tsarnaev, identified as Suspect No. 1, was killed overnight, officials said.
When Green's photo of one of the Boston bombing suspects fleeing the scene first surfaced, there was considerable doubt as to its authenticity because of the very low resolution of the image, which made the photo appear to be a composite image.
When Green later provided the high-resolution frame directly from his cellphone, editors of The Associated Press were able to establish its authenticity based on the improved resolution as well as the time the photo was taken. The AP has established an exclusive arrangement for distribution of the photograph.
Green, back at his home in Florida, wore his yellow and blue Boston Marathon jersey as he talked about the now-famous photo, his finisher's medal from the race propped on a shelf in his home office.
Green, 49, had finished Monday's marathon in 3 hours and 17 minutes, about an hour before the blasts. An avid endurance athlete and CEO of 110% Play Harder, a sportswear company, it was his first Boston Marathon and four of his co-workers had come to the finish line to cheer for him.
After he recovered, he went back to Boylston Street, where the finish line is located, to watch the rest of the race with his friends. He realized his phone was dying, so he went into a nearby store with a recharging station.
About 15 minutes later, he was walking back to his friends when the first bomb went off.
"I thought maybe it was a cannon," Green said. Then the second one exploded as he was walking toward it.
"When I saw it, I pulled out the camera and immediately took that picture," Green said.
He then put it back in his pocket and went to help the injured, including a boy and others who were missing limbs.
"It was like battle - a lot of noise, a lot of smoke, people coming at me in a panic," he said.
A short time later, his friend Jason Lubin texted him and asked if he was OK. He replied with the photograph and a note: "It was just in front of me."
Lubin said Thursday night, after the FBI released photos of the two suspects, that he decided to take a closer look at Green's photograph - on the off chance Green had captured anything unusual. He pulled up the photo on his smartphone and zoomed in on the crowd. There in the lower left corner was Tsarnaev walking around a corner, his backward white baseball cap standing out amid the dozens of panicked people fleeing. He appeared to be buttoning his jacket.
"I literally had to sit down," Lubin said.
He said he immediately sent Green a text message.
"I told him in my view it was a 100 percent match with the guy on TV," Lubin said.
That prompted Green to contact the FBI, which told him to send them a copy of the photograph.
He said that looking at the photo now, he is struck by two things: Tsarnaev isn't carrying the backpack the surveillance videos show him with before the bombings, and he is acting very differently from everyone around him.
"He is calmly walking, without panic," Green said.