Connecticut authorities have not been able to determine what prompted gunman Adam Lanza to carry out his 2012 massacre at Sandy Hook Elementary School, an attack that shattered the bucolic community of Newtown and thrust the country into a long, soul-searching debate over gun control and mental-health care.

Nearly a year after Lanza killed his mother, 20 children, six educators and himself, investigators have concluded that there is no evidence that points to "a conclusive motive," according to a report released Monday.

In other words, the case's most burning question — Why? — may never be answered.

There is also no clear explanation for Lanza's choosing the school as his target and the children and adults inside as his victims. It may have been simply that he lived nearby, investigators said.

"In fact, as best as can be determined, the shooter had no prior contact with anyone in the school that day," the report said.

The 43-page document, which condenses thousands of pages included in the state police's file on the case, said that Lanza, 20, acted alone, and suffered from various mental health issues, including Asperger syndrome. Investigators concluded that Lanza's mental state provided "no defense to his conduct," saying that the evidence showed "his intentions to kill" and that he understood that what he was doing was illegal.

"It is clear that the shooter planned his crimes in advance and was under no extreme emotional disturbance for which there was a reasonable explanation or excuse," the report said.

The report concluded that no criminal charges will be filed in the case, in which Lanza — who stood 6 feet tall and weighed 112 pounds — killed his mother at home then drove to the school and gunned down 26 people in the span of five minutes before committing suicide. It was the second-worst school shooting in American history.

An autopsy of Lanza's body found no drugs in his system, the report said. There was no evidence to suggest Lanza had taken any medication that would affect his behavior.

A piece of the story

The report was released Monday afternoon by Stephen Sedensky III, the state’s attorney for the judicial district of Danbury. It came less than three weeks before the anniversary of the Dec 14, 2012 attack, an event that will reopen emotional wounds among victims' families and expose the town to a new round of unwanted attention.

Sedensky has resisted calls by the media and First Amendment advocates to make public the entire stack of documents — and 911 calls. He has relied on input from victims' families, some of whom have demanded tighter restrictions on the release of investigative files.

He said Monday that he would release no additional information, presumably leaving it up to another agency to decide whether to release the remaining files. The state police has indicated that it may do so.

“With the release of this report today the investigation is closed, and no additional release of information or documents by this office is anticipated,” Sedensky said.

The family of Vicki Soto, a teacher killed by Lanza, said in a statement that the release of the report was "yet another blow" as the anniversary approaches.

Connecticut Gov. Dannel Malloy responded with a statement that expressed sympathy for the families of Lanza's victims, but also hope that the report, and voluminous backup documents, will be used to prevent similar crimes.

"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," Malloy said.

A troubled life



The report sketches out Adam Lanza's brief, troubled life, including his descent into mental illness.

His problems began as a young boy in the late 1990s, when he suffered from "speech and language needs." He also began having seizures.

In 2005, Lanza was diagnosed with Asperger syndrome, the report said, which involved "significant social impairments and extreme anxiety." An evaluation determined that he lacked empathy.

“As he got older, his condition seemed to worsen and he became more of a loner,” the report said. In middle school, he recoiled from "noise and confusion" and struggled to walk between classes. “As a result, in high school the shooter was home-schooled for a period of time.”

At the same time, Lanza developed a "preoccupation with mass shootings," particularly the 1999 murders at Columbine High School in Colorado, and "a strong interest in firearms."

Investigators searching the Lanza home discovered a spreadsheet tracking mass murders over the years, newspaper articles on shootings involving schoolchildren in 2008 and 1891 and a copy of the book "Amish Grace: How Forgiveness Transcended Tragedy," which chronicled a 2006 mass shooting at an Amish school in Pennsylvania that left five young pupils dead.

They also found a computer game depicting a school shooter, images of Lanza holding guns to his head and "two videos showing suicide by gunshot."

Puzzling contradictions

The report revealed many instances of contradictory behavior that made it difficult to understand why he killed. He was obsessed with guns and violence, but "displayed no aggressive or threatening tendencies" until Dec. 14. In some contexts he was considered intelligent, and in others below-average. Some acquaintances said he'd been bullied, and others said they saw no such problem. "With some people he could talk with them and be humorous; but many others saw the shooter as unemotional, distant, and remote," the report said.

Equally puzzling was his relationship with his 52-year-old mother, Nancy, his primary caregiver. Some people who knew him said he didn't have much emotional connection with her, or cared if something bad happened to her. Others told investigators they thought Lanza was close to his mother and that she was the only person to whom he would talk.

"Nancy Lanza didn't work because of her son's condition, and worried about what would happen to (him) if anything happened to her,” the report said.

A month or so before the massacre, Lanza's mother grew concerned about his reclusiveness, the report said.

In November 2012, she tried to buy him a computer, or parts of a computer, out of concern that "he hadn’t gone anywhere in three months and would only communicate with her by e-mail, though

they were living in the same house," according to the report.

But Nancy Lanza "never expressed fear of the shooter, for her own safety or that of anyone else."

That point is clear in her plans for Christmas: she wanted to buy him a CZ 83 pistol, "and had prepared a check for that purchase."

Nancy Lanza had a permit to carry a pistol, but Adam did not.

Studying shootings

In the months before the attack, Adam Lanza downloaded a significant amount of material related to mass shootings onto his computer, including video clips about Eric Harris and Dylan Klebold, the Columbine gunmen, surveillance footage of a Cleveland school shooting, a 2007 mall shooting in Omaha, Neb., and two videos showing suicide by self-inflicted gunshots, an appendix to the report said.

Lanza also had a file labeled "FUN" that included images of him holding guns to his head and a five-second video titled "Postal" that included a dramatization of children being shot.

He frequently posted on a blog that focused on mass shootings, including the Columbine attack. He also exchanged e-mails with others who were interested in the topic of mass shootings.

But none of those communications indicated what Lanza was going to do. And there was no evidence that any of those who traded messages with him played any kind of role in the murders, the report said.

Cache of weapons

Monday's report was the most detailed document officials have released since March, when a court seal on several search warrants expired. Those warrants described a massive cache of guns, knives, swords and ammunition found in the Lanza home.

The stockpile in the Lanza home included rifles, a BB gun, a starter pistol, several large-capacity magazines and a huge array of ammunition of various sizes. There was also a bayonet and a pole outfitted with a spear and blade.

Seized along with the weapons were photographs of what appeared to be a bloody body, a New York Times article about a 2008 mass shooting at Northern Illinois University, self-help books for understanding the minds of people with Asperger syndrome and autism, a guide to pistol shooting and a holiday card containing a check Nancy Lanza wrote to her son for the purchase of a firearm.

Investigators also took several of Adam Lanza's personal journals and drawings, a smashed hard drive, handwritten notes on the addresses of local gun shops and several printed emails.

The attack

Those documents, along with Monday's report, show that Lanza began his rampage by shooting his mother four times with a .22 caliber rifle while she lay in bed. He then drove her Honda Civic to nearby Sandy Hook Elementary School, parked in the fire lane and shot his way into the entrance. Once inside, he opened fire on the students and school workers with a .223 caliber Bushmaster rifle. As police approached, Lanza killed himself with a Glock 10 mm handgun.



The first 911 call from the school reached authorities at 9:35 a.m. Officers found the dead victims in two classrooms near the front door. In another classroom they came upon Adam Lanza's body, outfitted in a pale-green pocket vest, a black polo shirt, black cargo pants and black sneakers, the report said. He also wore yellow earplugs and black fingerless gloves on each hand.

The Bushmaster still had 14 rounds in its magazine and one in its chamber. Lanza also had several additional 30-round magazines for the Bushmaster, some spent and some still loaded, and a third gun, a loaded 9 mm Sig-Sauer pistol, which apparently was not used to shoot anyone.

The report released Monday said that the first police officer arrived at the school at 9:39 a.m., less than four minutes after the first 911 call. Lanza's final shot, killing himself, rang out at 9:40 a.m.

Lanza fired 154 rounds total, about half of the live ammunition he brought to the scene.

In the Civic, authorities recovered a 12-gauge Izhmash Canta shotgun, and two magazines containing 70 shotgun rounds.

Officers also found a rifle in the bedroom where Nancy Lanza died.

In all, police seized five guns that were “directly involved” in the killings. Nancy Lanza had bought them all legally.

Torey Van Oot, Daniel Macht, LeAnne Gendreau, Bob Connors, Ari Mason, An Phung, Sara Frazier and NBC News contributed to this report.