Both gunman Adam Lanza's parents and the education system worsened his social isolation in the years before he carried out the 2012 Sandy Hook Elementary School massacre by accommodating, rather than confronting, his difficulties, according to a report.
The Office of the Child Advocate, which investigated Lanza's upbringing to glean lessons for preventing future tragedies, concluded that his parents, education team and others missed signs of how deeply troubled he was and opportunities to steer him towards more appropriate treatment.
Lanza, 20, killed his mother then shot his way into the primary school in Newtown, Connecticut, on December 14 2012, and gunned down 20 children and six staff before committing suicide.
His obsession with firearms, death and mass shootings have been documented by police files and investigators previously concluded the motive for the massacre may never be known.
In exploring what could have been done differently, the new report focused on his mother Nancy, who backed her son's resistance to medication and from the 10th grade kept him at home, where he was surrounded by an arsenal of firearms and spent long hours playing violent video games.
"Mrs Lanza's approach to try and help him was to actually shelter him and protect him and pull him further away from the world, and that in turn turned out to be a very tragic mistake," said Julian Ford, one of the report's authors.
The authors said Lanza's parents tried to obtain help for him in a variety of ways, but did not know which path to take and appeared not to grasp the depth and severity of his disabilities.
His parents were divorced and Lanza had not seen his father for two years. After 2008, his parents did not appear to seek any mental health treatment for him and there was no sustained input from a mental health provider after 2006, according to the report.
The one provider that seemed to understand the gravity of his condition, the Yale Child Study Centre, evaluated him in 2006 and called for rigorous daily therapy and medication for conditions including anxiety.
At the time, a Yale psychiatrist warned there was a risk of creating a "prosthetic environment which spares him having to encounter other students or to work to overcome his social difficulties", according to the report.
The day after the evaluation, Nancy Lanza told the doctor by email that her son would not agree to any sort of medication and that he had been angered by the medic's line of questioning. The Yale recommendations went largely unheeded.
In the eighth grade, Lanza was placed on "homebound" status, though he later returned before finishing high school through a combination of independent study, tutoring and college classes. Along the way, the report said, there was no indication that the Newtown school system or the paediatrician co-ordinated with service providers regarding Lanza's mental health needs, according to the report, which referred to Lanza as "AL".
"Records indicate that the school system cared about AL's success but also unwittingly enabled Mrs Lanza's preference to accommodate and appease AL through the educational plan's lack of attention to social-emotional support, failure to provide related services, and agreement to AL's plan of independent study and early graduation at age 17," the report's authors wrote.
Joseph Erardi, the superintendent of schools for Newtown, said the report would have great meaning if "there is one school leader, one district, one mental health provider or one set of parents who reads this work and can prevent such a heinous crime".
The report also provocatively asks whether a family that was not white or as affluent as the Lanzas would have been given the same leeway to manage treatment for their troubled child.
"Is the community more reluctant to intervene and more likely to provide deference to the parental judgment and decision-making of white, affluent parents than those caregivers who are poor or minority?" the report said.
Despite disturbing, violence-laced writings that came to the attention of teachers, investigators say there is no evidence Lanza displayed tendencies for violence or aggression.