A shattered window is seen in December 2012 at Sandy Hook Elementary School in Newtown, Conn. Adam Lanza gunned down 20 first-graders and six educators with a semi-automatic rifle at the school on Dec. 14, 2012, after killing his mother inside their home.

NEW HAVEN, Conn. — Connecticut police released thousands of pages Friday from their investigation into the Newtown massacre, providing the most detailed and disturbing picture yet of the rampage and Adam Lanza’s fascination with murder, while also depicting school employees’ brave and clearheaded attempts to protect the children.

Among the details: 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 Dec. 14, 2012, shooting at Sandy Hook Elementary 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.

Last month, prosecutors issued a summary of the investigation that portrayed Lanza as obsessed with mass murders and afflicted with mental problems. But the summary said his motive for the massacre was a mystery and might never be known.

In releasing the huge investigative file Friday, 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. The documents also indicated Lanza was interested in mass killings, played violent video games and had books that dealt with death.

A former teacher of Lanza’s was quoted as telling investigators that Lanza exhibited anti-social behavior, 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 in the math lab, heard Thorne yell, “Put the gun down!” An aide said she heard gunfire and that Thorne told her to close her door. Thorne 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 bathroom door as a barricade and then locked the door.