The gunman who killed more than two dozen people, most of them children, at an elementary school in a quiet Connecticut town forced his way into the building.
Adam Lanza, 20, shot his mother Nancy at their home, then drove to the school in her car and opened fire with two semi-automatic handguns and an assault rifle.
Minutes later 28 people were dead - 20 children and six adults at the school, the gunman's mother at home, and Lanza himself.
Police said yesterday they had found evidence at the school and at the house. "Our investigators did produce some very, very good evidence … that they will be able to use in hopefully painting the complete picture as to how, and more importantly, why, this occurred," said Connecticut state police spokesman Lieutenant Paul Vance.
He said a woman who survived Friday's shooting will play an important role in the investigation.
The bodies of the dead were removed from the Sandy Hook Elementary School overnight and by early yesterday morning the medical examiner completed the formal identification of the victims.
Police asked the media to allow the families of the victims to "maintain solitude".
Details of the school massacre, the second deadliest in American history, began to emerge yesterday from witnesses.
Victoria Soto, a first-grade teacher in a classroom next to the one where most of the children were killed, hid her pupils in a bathroom. Moments later Lanza entered the room and, finding only Soto, shot her dead.
Brendan Murray, 9, was in the gym with his fourth-grade class when they heard loud bangs. "We thought that someone was knocking something over. And we heard yelling and we heard gunshots. We heard lots of gunshots," the boy said. "We heard someone say, 'Put your hands up!' … I heard, 'Don't shoot!' We had to go into the closet in the gym."
A first-grade teacher who had barricaded her students in her classroom's bathroom said she thought the gunman was coming for them next. "I wanted them to know that someone loved them and I wanted that to be one of the last things they heard, not the gunfire in the hallway," she said.
Police have not been able to identify a motive or even what Lanza's connection to the school was. Reports had identified his mother as a teacher there, but investigators have found no record that Nancy Lanza worked at Sandy Hook.
The weapons used in the shootings are believed to have been registered to Lanza's mother, who was found dead from gunshot wounds to the face. Sources described her as a "gun enthusiast said to be strict and very rigid".
There were reports that Lanza had a history of mental illness and was described by former classmates as a shy young man.
A former classmate, Andrew Lapple, who played Little League baseball with Lanza, said he "never really talked" and was a "tech geek".
"He was always carrying around his laptop holding on to it real tight," Mr Lapple said. "He walked down the halls against the wall almost like he was afraid of people. He was definitely kind of strange but you'd never think he'd do something like this."
Barack Obama addressed the country again yesterday after giving a statement on Friday in which he struggled to maintain his composure and wiped away tears.
"I know there is not a parent in America who does not feel the same overwhelming grief that I do," Mr Obama said.
The president also called for putting aside politics to address the recurring horror of gun violence in America.
While the president did not call for any specific action, the latest tragedy sparked a discussion in the news and social media about reform of America's gun laws. There have been at least 13 mass shootings in the country this year.
"This is America's Dunblane," the CNN host Piers Morgan wrote on Twitter. The March 1996 school shooting in a small Scottish town killed 16 children and their teacher. "We banned handguns in Britain after that appalling tragedy. What will the US do? Inaction not an option."
Statements of grief and disbelief came from around the world yesterday.
In a letter to the Roman Catholic church in Newtown that was read at a vigil there on Friday, the Vatican said Pope Benedict XVI conveyed "his heartfelt grief and the assurance of his closeness in prayer to the victims and their families".
The British prime minister David Cameron said: "My thoughts are with the injured and those who have lost loved ones. It is heartbreaking to think of those who have had their children robbed from them at such a young age, when they had so much life ahead of them."
The Philippines president Benigno Aquino said he and the Filipino people stood beside the United States "with bowed heads, yet in deep admiration over the manner in which the American people have reached out to comfort the afflicted, and to search for answers that will give meaning and hope to this grim event".
In Baghdad, Hassan Sabah, a shop owner, said: "We feel sorry for the victims and their families. This tragic incident shows there is no violence-free society in the world, even in the western and non-Muslim countries."