Doug Stanglin and Marisol Bello

USA TODAY

A Connecticut agency investigating the 2012 Sandy Hook school massacre finds that the killer, Adam Lanza, did not "snap" but was obsessed with mass murder and had carefully planned the attack.

Lanza, 20, killed his mother at their home in Newtown, Conn., on the morning of Dec. 14, then went to the school he had once attended and gunned down 20 children and six staff members before killing himself.

The report, issued Friday by the Office of Child Advocate, said Lanza, over the years, had become "increasingly preoccupied with mass murder" and was encouraged by "a micro society of mass murder enthusiasts with whom he was in email communication."

The report did not offer any specific motive for the killings, but noted that his mother was planning to leave the area and that he may have been "stimulated by fears of leaving the 'comfort zone' of his home."

The investigation delved deeply into Lanza's past, noting that his mental condition had gotten worse in his 10th grade, when he withdrew from most classes. It said he "progressively deteriorated in the last years of his life, eventually living in virtual social isolation."

At the time of his death, the 6-foot tall Lanza was anorexic, weighing only 112 pounds, the report said.

A Connecticut judge last year ordered Newtown school officials to give Lanza's records to the child advocate agency for its investigation.

The governor's Sandy Hook Advisory Commission has been waiting for the office's report before releasing its recommendations on what the state can do to prevent and respond to future incidents.

Friday's study is the second comprehensive report released by the state about the Newtown killings. The state's attorney's office released a report in November, 2013, detailing the events of the killings on Dec. 14, 2012 and the weeks leading up to it.

That report also provided insight into Lanza's mental state, but investigators could not identify a motive for the killings.

Friday's report was an effort identify public health recommendations based on a review of Lanza's personal history. The report found that there were early indications about Lanza's obsession with violence that were ignored or were not taken seriously by his parents, medical professionals or the school district.

The report recommends more thorough mental health screenings by doctors and schools, better coordination of care so that doctors, therapists and school administrators are aware of the child's needs and more support for families with children facing mental health issues.