Behind the news of the marathon: data crunching and photo verification

Personal tales of Boston marathoners came together after The Associated Press analyzed images and data, including the finishing times recorded by chips on competitors’ bibs, to pinpoint some of the runners who were in the finish line area when the bombs went off.

AP has distributed photos taken by Massachusetts engineer Bob Leonard, whose images near the finish line clearly show the two brothers suspected in the April 15 blasts, 26-year-old Tamerlan Tsarnaev, who was killed in shootout with police, and 19-year-old Dzhokhar Tsarnaev, later captured. “They actually stood in that corner for quite a bit of time,” Leonard told AP.

Leonard was not the only picture-taker to help with images of the suspects. David Green, a businessman from Jacksonville, Fla., who ran the marathon, produced a photo in which Dzhokhar Tsarnaev appears fleeing the scene, though initially there was doubt as to its authenticity because of the very low resolution of the image. It made the photo appear to be a composite image.

When Green later provided the high-resolution frame directly from his cellphone, editors of the AP 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.

A story about Leonard and Green includes a slideshow with 12 of their revealing images.