Shortly after finishing the Boston Marathon this week David Green, 49, was walking to meet friends when two bombs exploded in front of him as he faced east on the corner of Fairfield and Boylston Streets. He snapped a photograph with his iPhone before rushing to help those wounded. It was time-stamped at 2:50:15 p.m.

He posted it to Facebook with an account of his experiences.

On Thursday, when the FBI released images of the two suspects in the blast, Green’s friends began to notice a figure in a white hat in the bottom left corner of his picture that closely resembled one of the suspects.

One posted it in a running forum, from where it went viral, and sparked a debate over its veracity.

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Late Thursday, Green, who runs a sportswear company in Florida, confirmed that the photo is genuine, and that the FBI had contacted him and requested a copy.

Agents had told him, he said, that his picture was the best they had, because it was taken from a better camera than others that were posted.

He does not remember noticing the man in the white hat. He had, instead, called the FBI earlier to report another man, who was standing still and yelling instead of running.