Watch Raising Adam Lanza on PBS. See more from FRONTLINE.

How do you make sense of a seemingly senseless act of violence? How do you help the country begin to process the trauma of 20 small children shot dead in their classroom?

The Hartford Courant and Frontline are piecing together the lives of Nancy Lanza and her son Adam, who killed his mother and 26 first-graders, school officials and teachers during the massacre at Sandy Hook Elementary School in December.

Adam attended Sandy Hook as a first-grader, but his mother pulled him out of the school and several other schools over the course of his childhood.

In addition to Asperger’s syndrome, Adam from an early age also had sensory integration disorder, which left him unable to handle loud noises, pain and crowds, but is not a universally accepted medical diagnosis.

As a child, Adam got upset when others gave him a high-five or a pat on the back. It saddened his mother, Nancy, who didn’t know how to help him.

Andrew Julien, editor of The Hartford Courant, points out: “Nancy Lanza is the person Adam was closest to in the world. She was the first person he killed. He shot her four times in the head while she was in bed, and then he went off to Sandy Hook Elementary School. If we can begin to understand Adam’s relationship with Nancy, we probably can begin to understand Adam.”

Part one, Raising Adam Lanza, draws on Nancy’s own emails, previously unseen photos and exclusive home video footage of Adam, as well as insider interviews, to reveal a mother’s complex relationship with her troubled young son. Part two, Newtown Divided follows Courant reporter Matt Kauffman as he explores the consequences of the shooting in a town that has a long history of firearms and gun ownership, and where people most deeply affected by the tragedy are wrestling with our nation’s gun culture and laws.

WFSB 3 Connecticut

The ATF raided a gun store in East Windsor, Connecticut on Thursday evening after 26-year-old Jordan Marsh, who you can see in the video above stealing a rifle, was found to have an AR-15 with a scope in his possession at the Hartford Hilton on Saturday. Marsh did not pay for the gun, he simply took it off the rack and walked out of the store without anyone realizing it. Marsh reportedly has a history of mental illnesses and police believe he was planning to carry out an attack "similar" to that of Adam Lanza.

The gun store, Riverview Gun Sales, is also the same store where at least one of the guns used by Adam Lanza at the Sandy Hook Elementary School shootings was purchased by his mother, Nancy Lanza.

The AR-15 was banned under the now expired Federal Assault Weapons Ban of 1994.

WFSB 3:

Police said Riverview Gun Sales had no idea the AR-15 Marsh stole was missing. Management at the store didn't know about 11 guns that Marsh had allegedly stolen last year until they were notified by detectives.

Inventory control issues at Riverview Gun Sales have occurred before. In 2007, state police raided a Somers home and found a bunch of stolen guns from the store.

"It was found that the same Riverview gun store was missing upwards of 30-plus guns," said East Windsor police Detective Matthew Carl.

[Emphasis mine.]

Hopefully, the ATF took the store's federal firearms license along with them when they left.