Thousands of documents related to police investigation into the killing of 26 people at Sandy Hook Elementary School in Connecticut have been released.
Officials described as “painful” some of the details in the report, which includes photos and transcripts.
The report reveals new facts about the life of gunman Adam Lanza, 20, said to have suffered from mental health issues.
Adam Lanza turned a gun on himself after opening fire at Sandy Hook Elementary School in Newtown in December 2012.
Before killing 20 children and six adults with a semi-automatic rifle, Adam Lanza shot dead his own mother at the family home. She had bought him the weapons used in the rampage.
Last month, Connecticut police published a summary of their findings, showing that Adam Lanza had “an obsession” with the 1999 Columbine massacre, during which two teenagers killed 12 students and a teacher.
Investigators also found that Adam Lanza had acted alone and planned the shooting, which took less than 11 minutes.
The final report contains evidence that Adam Lanza’s mother discussed her son’s “disabilities” with a friend they day before the Sandy Hook attack.
The student would “write 10 pages obsessing over battles, destruction and war”.
The findings also reveal autopsy reports and new information about how the shooting unfolded.
A teacher hiding in a closet said he heard janitor Rick Thorne challenge Adam Lanza. Rick Thorne, who survived the attack, apparently yelled: “Put the gun down.”
Among the audiovisual material released on Friday were photos of items found in Adam Lanza’s home, including numerous rounds of ammunition, gun magazines, shooting earplugs and a gun safe with a rifle in it.
The files were heavily redacted to protect the students’ identity and to withhold gruesome details of the crime.
The Newtown shooting prompted a renewed US campaign for stricter firearms controls.
While no legislation was passed at a national level, some states – including Connecticut and Colorado – imposed tougher gun laws. Other states loosened restrictions.