They follow the cell # and the users whereabouts at different times before the bombing. They can find a photo of he guy in a different place and time and match the recorded signal and history of the phone.
All cell tower logs show is that somebody made a call to a certain number. Big effin’ deal. Its not against the law to make a call just before, during, or right after a bombing. Besides, if really was the bomber, he should have used that phone exactly once, then into the harbor with it...
Now if they can use the image to track him him back and get pictures of him dropping off some packages, they’ll have something! Maybe they can use face recognition software?