A state investigation of last year’s elementary school massacre in Newtown, Conn., concluded that the shooter acted alone and had “significant mental health issues,” but investigators were unable to determine conclusively a motive for the attack or why he targeted the school.

A 48-page report released Monday by Connecticut’s lead investigator in the case, State’s Attorney Stephen J. Sedensky III, said that Adam Lanza, 20, shot his mother four times with a .22-caliber rifle as she lay in bed in their Newtown home, then drove to Sandy Hook Elementary School, which he had once attended. There, he forced his way in and used a Bushmaster XM15-E2S semiautomatic 5.56mm rifle to kill 26 more people — 20 children and six adults — and wound two other adults before committing suicide with a Glock 10mm handgun as police closed in.

Lanza “acted alone and was solely criminally responsible for his actions of that day,” the report said. “Moreover, none of the evidence developed to date demonstrates probable cause to believe that any other person conspired with the shooter to commit these crimes or aided and abetted him in doing so.”

Therefore, it said, “there will be no state criminal prosecution as result of these crimes” and “the investigation is closed.”

The report added: “The obvious question that remains is: ‘Why did the shooter murder twenty-seven people, including twenty children?’ Unfortunately, that question may never be answered conclusively, despite the collection of extensive background information on the shooter through a multitude of interviews and other sources. The evidence clearly shows that the shooter planned his actions, including the taking of his own life, but there is no clear indication why he did so, or why he targeted Sandy Hook Elementary School.”

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Sandy Hook

Connecticut report on the Sandy Hook Elementary School shootings

An investigation into the December 2012 massacre in Newtown, Conn., failed to determine the motive of shooter Adam Lanza.

Lanza “had significant mental health issues that affected his ability to live a normal life and to interact with others,” the report said. But he “did not recognize or help himself deal with those issues,” it said. “What contribution this made to the shootings, if any, is unknown as those mental health professionals who saw him did not see anything that would have predicted his future behavior.”

Lanza was familiar with and had access to guns and ammunition and had “an obsession with mass murders,” notably the April 1999 shootings at Columbine High School in Colorado, the report said. However, he gave no indication “that he intended to commit such a crime himself,” the investigators found.

Authorities had previously determined that Lanza fired 154 shots with the Bushmaster within five minutes. He took the weapon, the commercial version of the M-16 military assault rifle, and three other firearms from the home of his divorced mother, Nancy Lanza, who had assembled an arsenal in the house on the outskirts of Newtown.

“All of the firearms were legally purchased by the shooter’s mother,” Sedensky’s report said.

The Dec. 14 massacre prompted a national outcry, leading to calls for tighter gun laws and increased school security. President Obama urged Congress to pass legislation requiring universal background checks on firearms purchases, banning the sale of assault weapons and limiting the capacity of ammunition magazines to 10 rounds. But the National Rifle Association and its allies in Congress mounted a campaign against any new gun-control legislation, and a bill that would have expanded background checks was defeated in the Senate.

The long-awaited report by Sedensky, the Danbury state’s attorney, did not include the full state police evidence file. A Connecticut law passed this year in response to the massacre prohibits the release of photographs, video and other images of homicide victims if the material might invade their privacy or that of their surviving family members.

Sedensky has gone to court to fight release of the 911 tapes from Sandy Hook and has resisted calls from Connecticut Gov. Dan Malloy (D) to divulge more information sooner, the Associated Press reported. The news agency has waged a legal battle for the release of the tapes.

New Britain Superior Court Judge Eliot Prescott said Monday that he will listen to the 911 recordings before ruling on whether they can be publicly released, AP reported.

According to Monday’s report, the first Newtown police officer arrived at the school less than four minutes after the initial 911 call, which was received shortly after 9:35 a.m. A minute after the officer arrived, Lanza shot himself in the head, and police waited nearly five minutes more before entering the school.

At the house where Lanza and his mother lived, investigators found black trash bags taped over the windows of his bedroom and a computer room. A computer hard drive was intentionally damaged, and information from it has not been recovered.

However, the investigators found that Adam Lanza spent considerable time playing video games — both violent and nonviolent — and that he was preoccupied with mass shootings. Among the belongings found at the home was “a spreadsheet with mass murders over the years listing information about each shooting,” the report said.

Also found were two videos showing suicide by gunshot, a computer game called “School Shooting” in which the player controls a character who enters a school and shoots at students, images of Lanza holding a handgun and a rifle to his head, and a five-second video dramatization depicting children being shot. In addition, the report said, there were “materials regarding the topic of pedophilia and advocating for rights for pedophiles,” although “not child pornography.”

The report said that in 2005, Lanza was found to have Asperger’s syndrome, with “significant social impairments and extreme anxiety,” as well as a lack of empathy and “very rigid thought processes.” It mentioned that he had obsessive-compulsive disorder and “couldn’t stand to be touched.” But he refused to take medication or undergo behavioral therapy.

He had a cellphone but never used it, and stopped talking to his mother at some point, communicating with her only by e-mail even though they were living in the same house.

An autopsy found that Lanza, 6 feet tall and 112 pounds at the time of his death, had no drugs in his system.