In his research for the National Geographic Channel documentary “9/10: The Final Hours,” filmmaker Erik Nelson was missing one vital piece. While he had more than enough exterior footage of the two towers, he had very little of what the buildings looked like inside. Thanks to an internet miracle, one of his film researchers discovered a wealth of images on the Estonian photo site Fotki. Commemorating the anniversary of 9/11 this week, The New Yorker retells how Nelson tried to track down the mysterious man that took the beautiful photographs of the empty interiors at night. The puzzle was finally solved, but unfortunately, the two will never be able to meet in person.