The result of a nearly yearlong investigation into the circumstances surrounding the massacre of innocents at Sandy Hook Elementary School confirms the troubling reality that presented itself within days of the Dec. 14 shooting.

A deeply troubled, mentally ill young man had access to battlefield-type firearms designed to kill quickly, effectively and massively. For reasons defying explanation or investigation, he murdered his mother and then turned an elementary school into a killing field, leaving 20 first-graders and six educators dead.

Danbury State's Attorney Stephen J. Sedensky III officially closed the investigation Monday, releasing a 44-page report summarizing the findings. There were no surprises.

Twenty-year-old shooter Adam Lanza acted alone.

Investigators could not determine a motive for the attack.

There were no earlier acts of violence by Mr. Lanza foreshadowing the events of Dec. 14.

Armed with a .223-caliber Bushmaster AR-15 semiautomatic rifle and carrying 31 pounds of ammo, Mr. Lanza killed so quickly that police had no reasonable chance to intervene. He killed himself with a handgun at 9:40 a.m., less than 5 minutes after the initial 911 call and 1 minute after the arrival of the first police.

He still had 253 rounds of ammunition on him at the time of the suicide.

The report provides further evidence of how disturbed an individual Mr. Lanza was and that the condition was long standing. A storybook he wrote as a 10-year-old fifth-grader is filled with disturbing images of armed and violent characters - including one who threatens to hurt and kill children in a classroom and another who declares, "Let's hurt children."

Family and educators recognized his problems. He received counseling. A doctor diagnosed Mr. Lanza with Asperger's syndrome in 2005, according to the report. He also showed signs of acute obsessive-compulsive disorder. Once a young adult, he apparently refused medication or to take part in therapy.

In the last couple of years of his life, the behavior grew more bizarre. He spent much of his time in a Spartan bedroom, sunshine sealed out by black plastic bags taped to the windows. Mr. Lanza became obsessed with the subject of mass shootings, particularly at schools, exchanging emails on the topic and blogging about it.

In the darkened room he spent much time playing violent video games, including a game titled "School Shooting" in which the player controls a character who enters a school and shoots students.

The report provides some insight into why the killer's mother and first victim, Nancy Lanza, indulged her disturbed son's fascination with firearms. Guns were something through which she could still have a relationship with him. They went to shooting ranges and took a National Rifle Association safety course together. This shared interest continued to the end. Investigators found a check written by her for him to purchase a pistol.

Investigators say son and mother were communicating otherwise only through emails and texts.

Gun advocates will focus on the mental health issues, but those issues were recognized and until Dec. 14 Mr. Lanza had yet to do anything - in the eyes of the law - to suggest he was a threat to himself or others, the threshold for an order of detainment due to mental illness.

This tragedy comes down to his access to guns of mass destruction. At a scrawny 112 pounds he was not much of a physical threat. With his Bushmaster AR-15 he was a killing machine.

If anything, this report vindicates the passage of the tough new gun laws in Connecticut, broadening the definition of assault weapons that cannot be sold in the state, prohibiting the sale or purchase of high-capacity magazines like those used in the Newtown shooting, and requiring more in-depth background checks for all gun purchases.

The public will probably never know precisely why Mr. Lanza did what he did, but it knows how he was able to do it.