[Editor’s Note: this is an update to an article originally published November 26 following the release of the Sandy Hook summary report.]

After Adam Lanza murdered his mother with a bolt-action .22 rifle as she lay in her bed, there was nothing to stop him from stealing her weapons and attacking Sandy Hook Elementary School. There is strong evidence, however, that an armed staffer could have ended Lanza’s rampage at the school after it began, potentially saving the lives of students and teachers.

To understand how an armed staffer could have saved lives, it is important to grasp the layout of Sandy Hook Elementary School, and the chronology of Lanza’s attack.

Here is a map the relevant quadrant of the school taken from the Sandy Hook summary report (PDF).

The fatalities at Sandy Hook elementary school took place in and around rooms 8, 9, and 10, which branch off from the hallway at the front of the school. Adam Lanza gained access to the school lobby by simply shooting his way through a plate glass window. He then turned left and began his attack.