Connecticut state police released thousands of pages covered with witness statements Friday.
The report includes previously unseen photographs, including blood-stained classroom floors, halls, and shell-casings on the floor of Sandy Hook Elementary School, where 20 students and 6 educators were fatally shot by Lanza on Dec. 14, 2012.
Lanza killed his mother, Nancy, and then drove to the school where he carried out his rampage. He killed himself as police arrived.
The paperwork, photos and videos were heavily redacted to protect the names of the children and to withhold some of the more graphic details of the crime.
Photographs from inside the Lanza family's home were also released and depict an undisturbed scene of upscale suburban living. Books were shelved in their places and graduation photos hung proudly.
At the house, investigators found books about autism and Asperger's syndrome as well as an NRA guide to pistol shooting.
The narrated police video also show a closet full of board games, numerous rounds of ammunition, gun magazines, shot-up paper targets, gun cases, shooting earplugs and a gun safe with a rifle in it.
Authorities also found nine knives and three swords of varying length. They found a military-style uniform in Lanza's bedroom and handwritten notes containing the addresses of local gun shops.
Investigators found articles on other shootings and a holiday card containing a check made out to Adam Lanza for the purchase of a firearm, authored by his mother, Nancy.
A separate photograph shows a gun on the floor of his mother Nancy's bedroom, where Lanza shot her to death.
In Lanza's bedroom, the video shows garbage bags covering the windows as well as his collection of video games and images of the computer police say he smashed.
An unnamed source told investigators that Lanza was an avid gamer who played "Call of Duty" and other games and rarely left his home.
By smashing the computer and destroying the hard drive, officials say the suspect destroyed a potentially key piece of evidence that could have helped investigators learn about how the 20-year-old planned the massacre as well as his motive.
The documents also fill in more details about how the shooting unfolded, teachers protected their students and the school janitor confronted the shooter.
Teachers heard janitor Rick Thorn try to get Lanza to leave the school. One teacher, who was hiding in a closet in the math lab, heard Thorn yell, "Put the gun down!" An aid said she heard the gunfire and Thorn told her to close the door.
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.
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.
The documents released insight into Lanza's emotional issues and his obsession with mass murders, but concluded that his 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 Associated Press contributed to this report.