Among the details: More than a dozen bodies, mostly children, were found packed "like sardines" in a bathroom where they had hidden. And the horrors encountered inside the school were so great that when police sent in paramedics, they tried to select ones capable of handling what they were about to witness.
"This will be the worst day of your life," police Sgt. William Cario warned one.
The documents' release marks the end of the investigation into the Dec. 14, 2012, shooting at Sandy Hook Elementary that left 20 first-graders and six educators dead.
Lanza, 20, went to the school after killing his mother, Nancy, inside their home. He committed suicide with a handgun as police arrived at the school.
Last month, prosecutors issued a summary of the investigation that portrayed Lanza as obsessed with mass murders and afflicted with mental problems. But the summary said his motive for the massacre was a mystery and might never be known.
In releasing the huge investigative file Friday, authorities heavily blacked out the paperwork, photos and videos to protect the names of children and withhold some of the more grisly details. But the horror comes through at nearly every turn.
Included were photographs of the Lanza home showing numerous rounds of ammunition, gun magazines, shot-up paper targets, gun cases, shooting earplugs and a gun safe with a rifle in it.
A former teacher of Lanza's was quoted as telling investigators that Lanza exhibited anti-social behavior, rarely interacted with other students and wrote obsessively "about battles, destruction and war."
"In all my years of experience, I have known (redacted) grade boys to talk about things like this, but Adam's level of violence was disturbing," the teacher told investigators. The teacher added: "Adam's creative writing was so graphic that it could not be shared."
The documents also fill in more details about how the shooting unfolded and how staff members looked out for the youngsters.
Teachers heard janitor Rick Thorne try to get Lanza to leave the school. One teacher, who was hiding in a closet in the math lab, heard Thorne yell, "Put the gun down!" An aide said that she heard gunfire and that Thorne told her to close her door. Thorne survived.
Teacher Kaitlin Roig told police she heard "rapid-fire shooting" near her classroom. She rushed her students into the classroom's bathroom, pulled a rolling storage unit in front of the bathroom door as a barricade and then locked the door.
She heard a voice say, "Oh, please, no. Please, no." Eventually, police officers slid their badges under the bathroom door. Roig refused to come out and told them that if they were truly police, they should be able to get the key to the door -- which they did.
Others weren't so lucky.
Police Lt. Christopher Vanghele said he and another officer found what appeared to be about 15 bodies packed in another bathroom. So many people had tried to cram inside the bathroom that the door couldn't be closed, and the shooter gunned them all down, Vanghele surmised.
Vanghele also recalled another officer carrying a little girl in his arms and running for the exit. Vanghele ran with him through the parking lot as the officer repeated, "Come on sweetie, come on sweetie." The girl didn't survive.
In a letter accompanying the files, Reuben F. Bradford, commissioner of the state Department of Emergency Services and Public Protection, wrote that much of the report was disturbing. But he added: "In the midst of the darkness of that day, we also saw remarkable heroism and glimpses of grace."
Lanza was diagnosed in 2006 with "profound autism spectrum disorder, with rigidity, isolation and a lack of comprehension of ordinary social interaction and communications," while also displaying symptoms of obsessive-compulsive disorder, according to Dr. Robert A. King, a professor at the Yale School of Medicine Child Study Center.
But he also told investigators that he observed nothing in Lanza's behavior that would have predicted he would become a mass killer. Contacted by The Associated Press, King referred questions to the Yale University press office.
Peter Lanza, who was estranged from his son, told police that his son had Asperger's syndrome -- a type of autism. Autism is not associated with criminal violence. Among the images released Friday was a photo of a birthday card he'd given Adam, offering to take his son hiking or shooting, though it's not clear when it was sent.
Kathleen A. Koenig, a nurse at the Yale Child Studies Center, told investigators that Lanza frequently washed his hands and changed his socks 20 times a day, to the point where his mother did three loads of laundry a day.
The nurse, who met with Lanza in 2006 and 2007, said Lanza's mother declined to give him prescribed antidepressant and antianxiety medication after she reported that he had trouble raising his arm, something she attributed to the drug.
Koenig unsuccessfully tried to convince Nancy Lanza that the medicine was not responsible, and the mother failed to schedule a follow-up visit after her son missed an appointment, police said.
In the documents, a friend told police that Nancy Lanza reported that her son had hit his head several days before the shootings. And an ex-boyfriend told police that she canceled a trip to London on the week of the shooting because of "a couple last-minute problems on the home front."
She told a friend two weeks before the shootings that her son was growing "increasingly despondent" and had refused to leave his room for three months.
They only communicated by email, with the mother saying he told her he wouldn't feel bad if something happened to her. His isolation was so complete that he refused to leave his room during Superstorm Sandy, the report said.
Just before the shooting, Nancy Lanza was in New Hampshire. She told a lunch acquaintance there that the trip was an experiment in leaving her son home alone in Connecticut for a few days.
The documents indicate investigators were gentle in their questioning of children, interviewing youngsters only if they or their parents requested it. Some of the parents thought talking openly about the shooting and getting accurate information out would help their children heal.
After the interviews, the children were given a copy of Margaret Holmes' book "A Terrible Thing Happened" to help them deal with that they witnessed.
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