Inside, they moved in teams of three and four, guns drawn, sweeping rooms in a search for victims and survivors, still unaware of the horror that lay beyond the doors of two of the classrooms. It was smoky, with the acrid smell of gunpowder, and near the shooting scenes, officers had to step over spent shells that littered the floors.
The officers relied on training for the tactical rules of dealing with an active shooter. But nothing prepared them for the scenes inside classrooms 8 and 10.
Newtown Officer William Chapman knew that at least two adults had been killed at the school, but as he went from room to room, he initially found only empty spaces or groups of students and staff, frightened but unharmed. He approached Victoria Soto's classroom, expecting to find more of the same.
Chapman saw the Glock pistol first, and then Lanza's body, laying where he had shot himself. Chapman stepped inside, quickly scanning the room for a possible second shooter.
And then, he wrote, "my heart broke."
"I walked around the room saying to myself, 'no, no, no,'" Chapman wrote in his report.
The heartache only grew as he moved to the next room, where Lauren Rousseau was working as a substitute teacher. Like Roig, Rousseau had tried to protect her children by hiding them in the bathroom.
Chapman wrote that it appeared the teachers -- Rousseau and special education aide Rachel D'Avino -- "were actively trying to protect the children at all costs." But they died trying. And the image of the children in the bathroom, Chapman wrote, "was the most horrific thing that I had ever seen."
Chapman then seized on the idea that there must be a survivor. "I remember thinking 'someone has to be alive, the shooter is down, it's time to get people out of here.'"
He returned to Soto's room, moving from child to child, checking for a pulse. He found one in a young girl, and shouted for officers to provide cover while he brought her out of the school.
"I began running across the parking lot towards Dickinson Drive with the girl in my arms praying that she would live and telling her that she was safe, that Jesus loved her, and that I was protecting her," Chapman wrote.
The loss of so many children in Rousseau's and Soto's classrooms tested the resilience of many officers.
One state trooper literally went weak in the knees at the sight of the children. "I stepped out of the room and someone grabbed me and asked me if I was all right," he wrote. "I said no."
State Police Sgt. Brent Aiken said numerous fellow troopers were in shock as they left the school. The troopers "were all speechless and had a look of disbelief on their faces," he wrote.
When State Police Sgt. William Cario approached the bathroom in Rousseau's room, he was "initially unable to comprehend what I was looking at," he wrote. Cario fixated on one boy, and struggled to process the reality of so many other fallen children.
"The face of that little boy is the only specific image I have in that room," he wrote.
Cario tried to establish a count of the victims in the two classrooms, "but my mind would not count beyond the low teens and I kept getting confused," he wrote.
But Cario still had work to do. He was aware Natalie Hammond had been shot, and he returned to the meeting room, swiftly bandaging her hand and leg. He remembers trying to conserve medical equipment, thinking there might be other survivors in need of treatment.
Although Cario didn't think it was safe to evacuate everyone in the room, he put Hammond in a wheeled desk chair and brought her outside. Police were keeping ambulances a safe distance away, so Cario placed Hammond in another trooper's car and instructed him to drive up to the ambulance staging area.
Hammond repeatedly told Cario "Thank you" and "God bless you."
Later in the day, Cario bumped into Matthew Cassavechia, director of emergency medical services at Danbury Hospital, who said medical personnel would have to be given access to the school to formally declare the victims dead. Cario led Cassavechia and two other senior paramedics toward the building.
"I tried to prepare them for what they were about to see. I told them of the number of victims and the nature of the wounds," Cario wrote. "I told Cassavechia, 'This will be the worst day of your life.'"