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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 Federal Bureau of Investigation released images of the two suspects in the blast, Mr. 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.

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

In an interview late Thursday, Mr. Green, who runs a sportswear company in Florida, confirmed that the picture was genuine, and that the F.B.I. had contacted him and requested a copy. “I have spoken to several agents,” Mr. Green said, “and they have got the photos, they are picking up material from me, and I don’t think there’s any doubt.”

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

Mr. Green does not remember noticing the man in the white hat. He had, instead, called the F.B.I. earlier in the week to report another man, who was standing still and yelling instead of running.

Mr. Green provided a larger version of the image, which shows the suspect’s face more clearly.