Today federal investigators began removing the basement floor of a building in lower Manhattan one block from where Etan Patz lived 33 years ago. They're searching for his remains more than three decades after the 6-years-old disappeared while walking by himself for the first time to his school bus stop.

The little boy became one of the first to have his photo placed on the side of a milk carton. Ed Smart says his case changed the culture and procedures used to find missing children. Watch a preview of Anderson's interview with Smart and tune in for the rest of the conversation on AC360° at 8 and 10 p.m. ET tonight.