Police have released thousands of pages of documents from their investigation into last year's Sandy Hook massacre in Newtown, Connecticut, revealing disturbing new details about the rampage and killer Adam Lanza.

The release of the documents on Friday marked the end of the police investigation into the December 2012 shooting at Sandy Hook Elementary School, which left 20 young children and six educators dead.

A former teacher of Lanza's was quoted as telling investigators that he exhibited anti-social behaviour, rarely interacted with other students and obsessed in writings "about battles, destruction and war."

"I have known... boys to talk about things like this, but Adam's level of violence was disturbing," the teacher told investigators. "Adam's creative writing was so graphic that it could not be shared."

Lanza, 20, committed suicide with a handgun as police arrived at the school.

The newly released police documents, photos and videos were heavily blacked out to protect the names of children and to withhold some of the more grisly details.

Disturbing scenes

The documents revealed that the scene inside the school was so horrific 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 sergeant William Cario warned one paramedic.

The documents also contained new details about how the shooting unfolded and how staff members looked out for young students.

Teachers heard janitor Rick Thorne try to get Lanza to leave the school, shouting: "Put the gun down."

Teacher Kaitlin Roig told police she heard "rapid-fire shooting" near her classroom before she rushed her students into an adjoining bathroom, pulled a rolling storage unit in front as a barricade, and locked the door.

In another bathroom, police lieutenant Christopher Vanghele said he and another officer found what appeared to be about 15 bodies, mostly children, packed inside. He surmised that so many people had tried to cram inside that the bathroom door could not be closed, and the shooter gunned them all down.

'Glimpses of grace'

In a letter accompanying the files, Reuben Bradford, commissioner of the state's Department of Emergency Services and Public Protection, said despite the disturbing contents, the documents also underscored the resilience of teachers and law enforcement officials who worked to protect the school.

"In the midst of the darkness of that day, we also saw remarkable heroism and glimpses of grace," he wrote.

Lanza's estranged father, meanwhile, told police that Lanza had Asperger's syndrome, a type of autism that is not associated with violence, and exhibited symptoms of obsessive compulsive disorder.

A nurse who met with Lanza said he had several ritualistic behaviours, including frequently washing his hands and changing his socks 20 times a day.

The release of the police documents comes about a month after the state's Division of Criminal Justice released a report on Newtown concluding that Lanza had acted alone, and that his motive may never be known.