Danbury State’s Attorney Stephen J. Sedensky III released a 44-page summary Monday of the long-awaited report on the Sandy Hook school shooting, concluding, in part, that a motive for the massacre could not be established and that Adam Lanza killed himself about a minute after the first police officer arrived at the scene.

Lanza, 20, killed his mother, Nancy, in their Newtown home before heavily arming himself and driving to the Sandy Hook Elementary School, where he shot his way into the building with a Bushmaster semiautomatic rifle. He then killed 20 students and six adults before shooting himself in the head with a Glock 10mm pistol.

“Many people have asked why the shooter did what he did on December 14, 2012,” Sedensky wrote in conclusion of the report. “This investigation, with the substantial information available, does not establish a conclusive motive.”

The report concludes that Lanza acted alone, though the possiblity that there was more than one shooter “remained a consideration beyond December 14.” Authorities detained a man with something in his hand at the school in the moments after the shooting who turned out to be a parent with a cell phone. Reporters were detained in the woods near the school, but eventually cleared of involvement in the shooting.

The report delves into the private life of Lanza, who was obsessed with mass shootings, particularly the Columbine school shooting, and who had become increasingly reclusive and isolated in the months before the shooting. He had been diagnosed with Asperger’s Syndrome in 2005 and refused to take medication or engage in therapy.

In addition to the large number of firearms, swords and other items previously reported being seized at the Lanza home at 36 Yoganada St. in Newtown, the report describes video games, photographs and other evidence uncovered by investigators.

A GPS device located in Lanza’s bedroom indicated he had driven in the vicinity of some of the town’s schools, including Sandy Hook Elementary School, on the day before the shooting, but did not drive up to the school.

Nancy Lanza, who was found dead in bed in her second-floor bedroom, had written her son a Christmas check for the purchase of a CZ83 pistol, according to the report. Lanza had newspaper articles about school shootings, including one dating back to 1891, and photographs of a dead human, covered in blood and wrapped in plastic.

The vast majority of people interviewed had no explanation for Lanza’s actions, the report said, but a review of “electronic evidence and digital media” revealed he had a preoccupation with mass shootings and a strong interest in firearms.

“For example, there was a spreadsheet with mass murders over the years listing information about each shooting,” according to the report. Lanza had a computer game titled “School Shooting” where the player controls a character who enters a school and shoots students.” Investigators also found a five-second video depicting children being shot and pictures of himself holding handguns and rifles to his own head.

Lanza also liked to play “Dance Dance Revolution,” an interactive music video game, according to the report, and had often played the game at a local theater with an unidentified acquaintance who told investigators that Lanza was “capable of laughing, smiling and making jokes, though always in a dry fashion,” and that he never mentioned being bullied when growing up.

“In the course of their conversation, the shooter indicated he had an interest in mass murders and serial killing,” according to the report. They never discussed the topic at length and there were no conversations about weapons or shooting at a gun range.

Nancy Lanza, who legally owned all the guns used in the shooting and found in the home, had taken Lanza and his brother, Ryan, to a local shooting range over the years and had taken National Rifle Association gun safety courses with Lanza, according to the report. At the local range, Lanza was described as “quiet and polite.” The mother had a pistol permit; Adam Lanza did not.

Lanza had not spoken to his father, Peter Lanza, or his brother since 2010, according to the report. He spent most of his time alone, playing violent and nonviolent video games, according to the report. Sometimes he would not respond to emails for weeks and would frequently reformat the hard drive of his computer “as a way of staying off the grid and minimizing his Internet trace,” according to the report. Pictures from the home indicated that Lanza had covered the windows of his bedroom with garbage bags.

In November 2012, Nancy Lanza was concerned about her son and said he hadn’t gone anywhere in three months and would only communicate with her by email, even when they were both at home. The source of the information from the mother is not revealed in the report, but it indicates that she “never expressed fear of the shooter, for her own safety, or that of anyone else.”

The report concludes that a timely response by police, who went into the school with the knowledge that “someone inside might be waiting to take their lives,” along with the heroic actions of school staff helped save many children’s lives. The evidence indicates that Lanza planned his actions, including taking his own life. He committed 26 counts of murder with special circumstances at the school and the murder of his mother in addition to several other major crimes, including the attempted murder of two people who were shot inside the school and survived, according to the report.

The Dec. 14 incident resulted in sweeping reforms of the state’s gun laws, requiring background checks for all gun purchases, banning more than 100 types of so-called assault weapons and large-capacity magazines and calling for the registration of banned weapons and magazines already owned by members of the public.

It also spawned questions about school safety and discussions of how to prevent future shootings through mental health screenings. Gun control legislation failed at the federal level.

The shooting also resulted in laws limiting the release of crime scene photos and other information, and the ongoing debate about the release of the 911 calls from Sandy Hook. On Monday, New Britain Superior Court Judge Eliot Prescott said he plans to listen to the recordings, which remain sealed, and issue a decision on whether they will be released.

Gov. Dannel P. Malloy issued a statement about the release of the report.

“My thoughts today are with the people who lost a loved one at Sandy Hook Elementary School, as they have been nearly every day since the tragedy,” the statement said. “The release of this report will no doubt be difficult on them. But if there is one thing that I believe we must do, it’s that we must honor the lives that were lost by taking steps to protect ourselves from another horror like this. I hope that the information in this summary and in the supporting documents that will be released by the State Police takes us closer to that goal.”

U.S. Sen. Richard Blumenthal also issued a statement calling the report a “gripping, graphic reminder of the cruelty and brutality that caused this unspeakable tragedy, and a testament to the bravery and courage of scores of first responders.”

“My thoughts and prayers remain with the families of the 26 victims and the entire Newtown community; all have remained resilient and resolute in the face of unspeakable tragedy,” Blumenthal said. “My hope is that this report adds momentum to the push to prevent gun violence, and that it provides the families with some closure in the wake of their tremendous loss.”

The state police have assembled thousands of pages of documents during their investigation. It is unclear when the report will be released.

k.florin@theday.com