(CNN) -- In the years leading up to the December 2012 massacre at Connecticut's Sandy Hook Elementary School, Adam Lanza went from a merely shy pre-teen to a mentally ill recluse obsessed with school shootings.
But during that long descent, Lanza never gave anyone any indication that he would one day turn a gun on his mother and then storm his onetime grade school with a semiautomatic rifle, killing 20 first-graders and six adults, investigators reported Monday.
And so Connecticut authorities closed the book on the second-deadliest shooting in U.S. history with the motive still a mystery. Lanza shot himself at the end of his 11-minute rampage, and police found no sign that he "voiced or gave any indication to others that he intended to commit such a crime himself," according to a 44-page summary of the investigation, released Monday.
"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," the report states.
Though he had attended Sandy Hook from first through fifth grades, investigators found no sign the 20-year-old was targeting any student, teacher or other employee at the school.
"In fact, as best as can be determined, the shooter had no prior contact with anyone in the school that day," the report states.
Lanza "had significant mental health issues that affected his ability to live a normal life and to interact with others," the report states. "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."
The killings in Newtown, about 60 miles outside New York, happened less than five months after a similar bloodbath at a movie theater in Aurora, Colorado, outside Denver. Those mass slayings triggered a nationwide debate over gun violence, school safety and mental health -- a debate that produced some new restrictions on firearms in several states. But it also yielded a backlash against those laws by gun-rights advocates and only limited action on a federal level after a Republican filibuster blocked expanded background checks for gun buyers.
Various witnesses described a fifth-grade Lanza as quiet but bright: "He wouldn't necessarily engage in conversation, but wouldn't ignore one," the report states. He attended parties, enjoyed music and played the saxophone.
But the same year, according to investigators, Lanza produced something called the "Big Book of Granny" -- in which a woman armed with a gun in her cane goes on killing sprees with her son, with children sometimes the targets. The story was related to a class project, but apparently never was handed in to the school, the report notes.
"It can't be a red flag if nobody sees it," Casey Jordan, a criminologist at Western Connecticut State University in Danbury, told CNN's Erin Burnett OutFront.
By late middle school, Lanza "did not like noise and confusion and began to have issues when he had to walk to different classes," the report states. He didn't want to be in a crowd. He started receiving tutoring and home schooling. By ninth grade, he was "shutting himself in the bedroom and playing video games all day."
"He was so enormously isolated," Jordan said. "His mother was not allowed in his room. No one was. So this didn't happen overnight. This was years of him slowly withdrawing, and we have that history going back to fifth grade, sixth grade."
As a child, Lanza had seizures and washed his hands excessively. In 2005, he was diagnosed with Asperger's syndrome, with doctors noting he "lacked empathy" and showed "extreme anxiety and discomfort with changes, noise, and physical contact with others." In high school, where he took part in a school tech club, Lanza never spoke of violence, but "was also remembered for pulling his sleeves over his hand to touch something," the report states.
After the shooting, investigators found that Lanza had sorted out the details of school shootings and other mass murders in spreadsheets. Among the clippings he kept was a reprint of a story in The New York Times about a man who shot at schoolchildren in 1891, wounding several with a shotgun. His computer contained two videos depicting gunshot suicides, two pictures of Lanza pointing guns at his own head and movies depicting school shootings.
But while many of his video games were violent, others were not. For months before the killings at Sandy Hook, he would go to a movie theater on weekends to play the dance game "Dance Dance Revolution" for hours, the report recounts.
Lanza lived with his mother, 52-year-old Nancy Lanza, after his parents split up in 2001. Nancy Lanza "took care of all of the shooter's needs" and "worried about what would happen to the shooter if anything happened to her," according to the report.
It didn't sound easy: The shooter was particular about the food that he ate and its arrangement on a plate in relation to other foods on the plate," the report recounts. "Certain types of dishware could not be used for particular foods. The mother would shop for him and cook to the shooter's specifications, though sometimes he would cook for himself."
Nancy Lanza did her son's laundry every day, but was not allowed into his room -- "No one was allowed in his room," where the windows were covered with black plastic trash bags, the report notes. Adam Lanza "disliked birthdays, Christmas and holidays," forbidding his mother from putting up a Christmas tree: "The mother explained it by saying that shooter had no emotions or feelings."
He was not medicated: Lanza "did not drink alcohol, take drugs, prescription or otherwise, and hated the thought of doing any of those things," investigators found. An autopsy found no sign of drugs in his system at the time of the killings, the report states.
One person described Lanza's relationship with his mother as "strained," while another told investigators he didn't appear to have "an emotional connection to his mother." But others said Nancy Lanza "was the only person to whom the shooter would talk."
Lanza's mother "tried within her limits" to help her son live a normal life, Jordan said, but "we have a society that shames mental illness."
"The mother was overwhelmed, did not know what to do with him and did allow him to isolate," Jordan said. "She tried to bring him out with the one activity they had in common, which was going to the shooting range."
Nancy Lanza grew up with firearms and "thought it was good to learn responsibility for guns," the report states. Both she and Adam Lanza shot pistols at a local range, where Adam "was described as quiet and polite." There was a large but undisclosed number of weapons in the home, all of which had been purchased by Nancy Lanza.
On December 14, 2012, the morning after Nancy Lanza had returned from a trip to New Hampshire, her son shot her four times in the head with a .22-caliber rifle. Then it was off to the school where he once had been a relatively happy child, packing four other guns and nearly 500 rounds of ammunition. He fired more than 150 shots from a .223-caliber Bushmaster rifle before turning a 10mm Glock pistol on himself once police arrived, according to the report.
Monday's report is separate from a much longer evidence file that Connecticut State Police will release at an unspecified date. That cache will be "thousands of pages long," according to Connecticut State Police spokesman Paul Vance.
The documents will include witness statements, a timeline of events and background on Lanza, and Vance said he believes they will offer a motive. The file is still being reviewed, with witness names and other identifying information being redacted, and there is no scheduled date for its release, Vance said.
But the family of Victoria Soto, a teacher who shielded her students before being shot to death, said Monday's release is "yet another blow that our family has been dealt."
A statement from the family said, "While others search for the answer as to why this happened, we search for the how. How can we live without Vicki? How do we celebrate Christmas without Vicki? How do we go on every day missing a piece of our family? Those are the questions we seek the answers for. There is nothing in the report that will answer those for us."
Connecticut Gov. Dannel Malloy said the report's release "will no doubt be difficult" for the relatives of those killed at Sandy Hook.
"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," Malloy said. "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."
Victims' family members were informed of the report, said Mark Dupuis, a spokesman for Danbury State's Attorney Stephen Sedensky, whose office conducted the investigation.
"We are sensitive to the needs of the families, and those needs are being addressed," Dupuis said.
CNN's Susan Candiotti, Rande Iaboni, Rob Frehse, Chris Boyette and Ronni Berke contributed to this report.