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This image contained in the "Appendix to Report on the Shootings at Sandy Hook Elementary School and 36 Yogananda St., Newtown, Connecticut On December 14, 2012" and released Monday, Nov. 25, 2013, by the Danbury, Conn., State’s Attorney shows a weapon found at Sandy Hook Elementary School in Newtown, Conn. Adam Lanza opened fire inside the school killing 20 first-graders and six educators before killing himself as police arrived. (AP Photo/Office of the Connecticut State's Attorney Judicial District of Danbury)

Photos from the Newtown school shooting report

A yearlong investigation into the Newtown school shooting has provided disturbing insights into the gunman who killed his mother and then massacred 26 other people, but it still has not provided a motive.

Prosecutors closed the case Monday on the Sandy Hook Elementary shooting last December. Their report offered a chilling portrait of 20-year-old Adam Lanza as a loner who was obsessed with violence and bound by ritualistic behavior.

But they were unable to answer the question that everyone has been asking since the tragedy: Why? (AP)