Among the details: More than a dozen bodies, mostly children, were seen packed "like sardines" in a bathroom. And the horrors inside school were so terrible 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 School 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.
The paperwork, photos and videos were heavily blacked out to protect the names of children and to withhold some of the more grisly details. But the horror comes through at nearly every turn.
Included in the file were photographs of the home Lanza shared with his mother. They show 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 obsessed in writings "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 she heard gunfire and Thorne told her to close her door. Thorne survived.
Teacher Kaitlin Roig told police she heard "rapid-fire shooting" outside of the school, 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 closed and 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, mostly children, 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.
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 that it also showed teachers trying to protect their children, law enforcement officials putting themselves in harm's way, and dispatchers working calmly and efficiently.
"In the midst of the darkness of that day, we also saw remarkable heroism and glimpses of grace," he wrote.
Peter Lanza, who was estranged from his son, told police that Adam had Asperger's syndrome - a type of autism that is not associated with violence - and exhibited symptoms of being "slightly OCD," meaning obsessive compulsive disorder.
A former Newtown High student who was in Tech Club with Adam Lanza recalled him pulling his sleeves over his hands any time he was handed an object from someone.
A nurse at the Yale Child Studies Center who met with Adam Lanza said he had several ritualistic behaviors, including frequently washing hands and changing his socks 20 times a day, to the point his mother did three loads of laundry a day.
The nurse said that Lanza's mother declined to give him prescribed antidepressant and antianxiety medication and that she failed to schedule a follow-up visit after he missed an appointment.
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."
Prosecutors previously issued a summary of the investigation last month that portrayed Lanza as obsessed with mass murders, but the report concluded that Lanza's motives for the massacre might never be known.
Lanza "was undoubtedly afflicted with mental health problems; yet despite a fascination with mass shootings and firearms, he displayed no aggressive or threatening tendencies," it said.
The new files revealed chaos during the rampage, and contain sometimes-conflicting accounts from witnesses.
Lanza remained silent as he aimed and fired in Room 10, according to an officer who interviewed the mother of one of the surviving students. The woman said her son, who ran from the classroom, recalled the shooter kicking in the door and then firing.
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.