(NEW YORK) -- From the time he was little, Adam Lanza couldn't bear to be touched. By middle school, the chaos and noise of large, bustling classrooms began to upset set him. At 20, just before the Newtown, Conn., shootings, he was isolated and, the world would later learn, disturbed.
All this was revealed in "Raising Adam Lanza," an investigative report by the Hartford Courant in partnership with the PBS news program FRONTLINE, which aired Tuesday night.
Before the age of 6, Lanza had been diagnosed with a controversial condition, "sensory integration disorder" -- now known as sensory processing disorder -- according to the report.
Those with sensory processing disorder or SPD may over-respond to stimuli and find clothing, physical contact, light, sound or food unbearable. They may also under-respond and feel little or no reaction to pain or extreme hot and cold. A third form involves sensory motor problems that can cause weakness and clumsiness or delay in developing motor skills.
Whether SPD is a distinct disorder or a collection of symptoms pointing to other neurological deficits -- most often anxiety or attention-deficit hyperactivity disorder (ADHD) -- has been debated by the medical community for more than two decades.
No one will probably ever know why Lanza shot his mother four times in her own bed, then went to Sandy Hook Elementary School to slaughter six women and 20 first-graders before taking his own life on Dec. 14, 2012.
But this report, the most detailed account to date on his troubled life, paints a picture of a child coping with special needs and a mother, "devoted but perhaps misguided," struggling unsuccessfully to help.
"The most surprising thing for me was this sort of inwardness of Adam, a world view of someone that was afraid of the world," said show producer Frank Koughan. "He just reacted badly to the whole world and didn't want to be part of it. He was not some violent monster, except on one particular day, when he was exceedingly monstrous."
The investigative team interviewed family and friends of the shooter's parents, Nancy and Peter Lanza, and reviewed a decade's worth of messages and emails from his mother to close friends, describing her son's socially awkward behavior.
"Adam was a quiet kid. He never said a word," Marvin LaFontaine, a friend of Nancy Lanza, told them. "There was a weirdness about him and Nancy warned me once at one of the Scout meetings … 'Don't touch Adam.' She said he just can't stand that. He'd become teary-eyed and I think he would run to his mother."
In 1998, the Lanzas left their home in New Hampshire for Connecticut with Adam, who had already been diagnosed with the sensory disorder and was "coded" with an individual education plan, according to a family member who did not want to be identified by FRONTLINE.
"It was somebody well-placed who was completely in a position to know," said Koughan, 45, a veteran journalist who produced the film, Drop-Out Nation.
Adam didn't recognize pain, another feature of some types of SPD. He couldn't cope with loud noise, confusion or change, which would cause him to "shut down," according to the report.
"He'd almost go into a catatonic kind of state, which is another reason why in hindsight, he didn't seem like a threat to anybody," said Koughan. "He didn't lash out or beat up kids. He went within himself, until one day he didn't."
In middle school, according to an interview with Richard Novia, who served as security chief for Newtown schools and advised the tech club, Adam was diagnosed with Asperger's syndrome, a part of the autism spectrum. Novia told Nancy he worried about the boy being bullied.
Asperger's can be linked to or confused with SPD, but neither condition is associated with violence, mental health experts tell ABC News. But they suggest the complexity of anxiety and alienation in a child could shed light on other factors that may have set Adam up for a breakdown.
"What it sounds like to me is you have a kid who probably had what has been described really well as social anxiety," said Dr. Max Wiznitzer, a pediatric neurologist who specializes in development and behavioral disorders, including autism, at UH Rainbow Babies & Children's Hospital in Cleveland.
"These behaviors [described by the FRONTLINE report] could have set him up for being bullied and somewhat ostracized and made fun of by his peers," he said. "With a certain type personality, he could ruminate on it and get stuck. Maybe he got angry and took the wrong road. But why did he not go after his peers, not little kids?"
As for Asperger's syndrome, Wiznitzer said of Adam, "There is insufficient data … We don't know who diagnosed him."
Sensory processing disorder is a controversial condition. Just this year, its submission to the American Psychiatric Association for inclusion in the 2013 Diagnostic and Standards Manual (DSM V) was rejected.
Wiznitzer says there are no distinct criteria for diagnosing SPD and that it masks other distinct conditions like anxiety, obsessive compulsive disorder (OCD) and ADHD, which can be treated with conventional drugs or cognitive therapy.
As for Adam, he said, "You have no idea what it was like at home -- a person with extreme anxiety keeps very quiet in the outside world. Sometimes with anxiety-based behavior -- and I am not saying he had this -- if he had a change in how he was functioning, it could lead to a thought disorder."
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