This report raises, but cannot definitively answer, the question as to whether better access to effective mental health and educational services would have prevented the tragic events at Sandy Hook. It is important to emphasize that AL’s developmental condition and mental health cannot be seen as determinative of his extreme violent behavior.

In an e-mail Adam Lanza wrote to an unidentified person three days before the Sandy Hook killings, he discussed Aurora Shooting suspect James Holmes and Jared Loughner:

As far as the Holmies go…well, the .gif of him dancing on a llama was cute. I guess that’s all I can say about the whole Holmie thing since I can’t really relate to it. I don’t understand why there weren’t the “he’s just a poor misunderstood puppy who needs help” type flocking around Jared Loughner since that spiel ostensibly applied to him more than James Holmes. And speaking more generally, I don’t really understand why Aurora shooting was considered such a big deal all-around, as if such a thing had never happened before. It’s not like its 1984.

As to the effect of his parent's divorce, it includes this comment he made:

[I]n a 2006 interview with a child psychiatrist, AL stated that he attributed his parents’ divorce to the fact that they must have “irritated” each other as much as they irritated him.

The report indicates Nancy Lanza had some issues of her own. When Adam was in elementary school, she wrote letters to a friend (presumably John La Fontaine, who previously came forward with emails or letters) in which she said she had a terminal auto-immune disease, was trying to make sure her children and husband would be cared for, but she was terrified. But her medical records don't indicate any life threatening illness:

Despite Mrs. Lanza’s preoccupation with her health and concerns about her mortality, a review of her medical records from that time do not confirm a significant neurologic disorder, autoimmune disorder, or multiple sclerosis—the latter a diagnosis she sometimes indicated that she had.

A medical record from her July 1999 neurology follow up indicates that all testing was unremarkable. The record notes that Mrs. Lanza was experiencing “significant stress in her life related to her husband.” Additional medical testing was recommended along with “psychotherapy for [Mrs. Lanza’s] emotional issues.”

...There is no indication that Mrs. Lanza was provided a terminal diagnosis by doctors at any time.

...a 2012 medical record signed by Mrs. Lanza’s primary care physician indicated that the physician treated Mrs. Lanza for over eight years, had seen her many times, and had “never noticed or treated any symptoms of multiple sclerosis.” The medical form also stated that there were no related medical conditions or history....nor [according to the family] did autopsy results confirm the presence of findings consistent with MS.

She told LaFontaine her maternal grandfather died of the disease 7 weeks after being diagnosed.

Adam, the report notes, was severely anorexic:

AL was anorexic (six feet tall and 112 pounds), to the point of malnutrition and resultant brain damage....Anorexia, Obsessive Compulsive Disorder, and autism are conditions that individually increase the risk of suicide. Anorexia can produce cognitive impairment and it is likely that anorexia combined with an autism spectrum disorder and OCD compounded AL’s risk for suicide.

In discussing the final months of his life, the report says:

After an exhaustive review of records, emails, and conclusions drawn by law enforcement agencies, authors conclude that AL was not obviously psychotic in the time period leading up to the Sandy Hook shooting, though he had a history of depression and suicidal ideation that can be seen in his emails during 2011 and 2012.

The report says there is no "one thing" that was the tipping point. It also says he did not suddenly "snap":

Authors conclude that there was not one thing that was necessarily the tipping point driving AL to commit the Sandy Hook shooting. Rather there was a cascade of events, many self-imposed, that included: loss of school; absence of work; disruption of the relationship with his one friend; virtually no personal contact with family; virtually total and increasing isolation; fear of losing his home and of a change in his relationship with Mrs. Lanza, his only caretaker and connection; worsening OCD; depression and anxiety; profound and possibly worsening anorexia; and an increasing obsession with mass murder occurring in the total absence of any engagement with the outside world. AL increasingly lived in an alternate universe in which ruminations about mass shootings were his central preoccupation.

The attack on Sandy Hook Elementary appears to have been a purposefully thought-out and planned attack—AL did not just “snap.” He visited the school’s website on numerous occasions. He had looked at the student handbook and viewed security procedures at the school.

The report says:

[T]here is no connection in the literature between AL’s developmental profile and an increased likelihood of violent actions.
The likelihood of an individual with Autism Spectrum Disorder and/or severe problems with anxiety and obsessive compulsive tendencies committing an act of pre-meditated violence, much less one of AL’s magnitude, is rare. Individuals with these mental health or developmental disorders are more likely to internalize (that is, to feel distressed emotionally or to be confused, socially inappropriate or inept, and sometimes to harm themselves inadvertently or intentionally) than to externalize (that is, to act out aggressively so as to harm others).

The report concludes with this statement:

There is no way to adequately explain why AL was obsessed with mass shootings and how or why he came to act on this obsession. In the end, only he, and he alone, bears responsibility for this monstrous act.