Police released thousands of pages from their investigation into last year's massacre in Newtown, Connecticut, providing the most detailed and disturbing picture yet of the rampage.
The documents show Adam Lanza's fascination with murder, while also depicting school employees' brave attempts to protect the children.
Among the details were that more than a dozen bodies, mostly children, were discovered 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 December 14, 2012, shooting at Sandy Hook Elementary that left 20 pupils and six staff dead.
Lanza, 20, went to the school after killing his mother, Nancy, inside their home. He killed himself 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 report said his motive for the massacre was a mystery and might never be known.
In releasing the huge investigative file yesterday, the 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 behaviour, 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, heard him yell: "Put the gun down!"
An aide said that she heard gunfire and that Mr Thorne told her to close her door. He 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 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.
She 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 were not 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 could not be closed, and the shooter gunned them all down, Mr Vanghele surmised.
In a letter accompanying the files, Reuben Bradford, commissioner of the state department of emergency services and public protection, wrote that much of the report was disturbing.
But he said: "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".
He also displayed symptoms of obsessive-compulsive disorder, according to Dr Robert King, a professor at the Yale school of medicine child study centre.
But he also told investigators that he observed nothing in Lanza's behaviour that would have predicted he would become a mass killer.
Peter Lanza, who was estranged from his son, told police that Adam had Asperger's syndrome - a type of autism. Autism is not associated with criminal violence.
Among the images released yesterday was a photo of a birthday card he had given Adam, offering to take his son hiking or shooting, though it is not clear when it was sent.
Kathleen Koenig, a nurse at the Yale centre, told investigators that Adam 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 Lanza in 2006 and 2007, said his mother declined to give him prescribed antidepressant and anti-anxiety medication after she reported that he had trouble raising his arm, something she attributed to the drug.
Ms 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 her son had hit his head several days before the shootings.
And an ex-boyfriend told police she cancelled 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 would not feel bad if something happened to her.