Authorities in Connecticut released a final report on Friday outlining the shooting at Sandy Hook Elementary School in Newtown on Dec. 14, 2012, providing the most complete account to date for one of the nation’s most wrenching massacres.

It contained hundreds of photographs, hours of video and voluminous crime scene reports.

Care was taken to conceal the most graphic crime scene images. Yet painfully detailed descriptions accompanied the often-redacted pages, relaying eyewitness accounts from students and school staff members, and snapshots of emergency workers dashing to locked doors, at times straining to convince terrified teachers on the other side that they were officers.

There were new details concerning the gunman, 20-year-old Adam Lanza, who killed 20 first graders and six adults in the school and then killed himself; and his mother, Nancy, who told an acquaintance the day before the shooting that she had traveled to New Hampshire for a brief trip as “an experiment to allow Adam to stay at home alone for a few days,” the documents said. Mr. Lanza also killed his mother.

The report also included many photographs of the contents of their house, including one of a young child holding what appears to be a gun, with ammunition in his lap.