The gunman who killed 27 people, including 20 children, on Friday targeted a school to which he had no apparent connection — forcing his way in and spraying classrooms with a weapon designed to kill across a battlefield, authorities said.

On Saturday, law enforcement officials gave new details about the rampage of Adam Lanza, which ended with Lanza’s suicide. Their new narrative partially contradicted previous ones and made a baffling act seem more so.

Lanza’s mother, for instance, was not a teacher at Sandy Hook Elementary, after all. She apparently was unemployed. So it was still a mystery why her 20-year-old son — after dressing in black, killing his mother and taking at least three guns from her collection — then drove the five miles to a school where he was a stranger.

The part of the story that remained grimly, awfully unchanged was what Lanza did when he got there.

Authorities on Saturday released the names of those Lanza killed at the school, who ranged in age from 6 to 56. And the state’s medical examiner — speaking in sanitized, clinical terms — described the results of something deeply obscene: a semiautomatic rifle fired inside an elementary classroom.

Read the stories of the Newtown shooting victims

“I’ve been at this for a third of a century. And my sensibilities may not be the average man’s. But this probably is the worst I have seen,” said H. Wayne Carver II. Carver described the children’s injuries, which he said ranged from at least two to 11 bullet wounds apiece.

He had performed seven of the autopsies himself. A reporter asked what the children had been wearing.

“They’re wearing cute kid stuff,” Carver said. “I mean, they’re first-graders.”

On Saturday, this small New England town and the country played out what is now a familiar ritual: the dumbstruck aftermath of a young gunman’s massacre. Word came that President Obama would arrive Sunday for an evening interfaith service, repeating his role from Fort Hood, Tex.; Aurora, Colo.; and Tucson, Ariz. He would again be chief mourner.

In Connecticut, people who had known Adam Lanza described him as odd, nervous and withdrawn, and they searched their memories for signs they’d missed. Memorials went up. Politicians talked — a little more forcefully this time — about how someone needed to be brave enough to talk about guns and gun control.

And, in Newtown, they started funeral preparations. This time, the ritual was for lives so new that it seemed impossible to speak of them in the past tense.

“He was just in the wrong place at the wrong time,” said Rabbi Shaul Praver of Adath Israel, who said that his congregation lost a first-grade boy. “His little body could not endure so many bullets like that.”

On Saturday, law enforcement officials said that Lanza had entered the school by force sometime after 9:30 a.m. Friday. Sandy Hook’s principal, Dawn Hochsprung, had recently installed a new security system in which the school doors were kept locked all day starting at 9:30. But Lanza had apparently shattered the glass in a window or door.

Lanza was carrying at least three guns from a collection maintained by his mother, who friends said enjoyed target shooting. Lanza had two pistols, a Glock and a Sig Sauer.

But he apparently chose a larger weapon, a .223-caliber Bushmaster rifle, for much of the killing. This rifle fires one bullet for every pull of the trigger, and the unusually high speed of its round was designed to produce significant internal damage. Authorities said Lanza fired dozens and dozens of times in a spree that lasted minutes.

“All the wounds that I know of at this point were caused by the long weapon,” said Carver, the medical examiner.

He said he saw multiple wounds on the bodies of those he examined, and based on his conversations with colleagues, “I believe everybody was hit more than once.” None of the victims likely survived very long after being hit, Carver said.

When police arrived, Lanza was dead. So were Hochsprung and five other adults. So were 18 children. Two more were pronounced dead later at a local hospital.

Sixteen of the 20 children were just 6 years old. The other four were 7.

Later, when investigators went to the home that Lanza shared with his mother, Nancy Lanza was found dead there — the first victim of the killings and the last discovered.

On Saturday, authorities said they had “very good evidence” regarding Adam Lanza’s motives. But they didn’t say what that evidence was, and law enforcement officials said they had not found anything like a suicide note.

“No words can truly express how heartbroken we are,” Adam Lanza’s father, Peter Lanza, said in a statement released Saturday. “We are in a state of disbelief and trying to find whatever answers we can. We too are asking why.”

Around the country, advocates for stronger gun-control laws said they hoped that the shock of this crime would start a debate that other mass shootings had not. Still, with so little known about Adam Lanza and the guns he used, it was difficult to say what sort of law, precisely, was needed to prevent another shooting like Friday’s.

“If having dozens of people gunned down in an elementary school doesn’t motivate Washington to do even the easy things they can do, it’s not clear what will,” said Mark Glaze, director of Mayors Against Illegal Guns, a group chaired by New York Mayor Michael R. Bloomberg (I) that represents 750 mayors across the country.

Politics will come later, once the country has become used to the idea that this actually happened. In Newtown on Saturday, the shooting still seemed to dwell in the realm of the unthinkable.

“The emotions of yesterday were just absolutely overwhelming,” Monsignor Robert Weiss of St. Rose of Lima Roman Catholic Church in Newtown said in an interview Saturday with NBC News. “I don’t know if the reality has really settled in yet.’’

Weiss had accompanied police when they notified parents that their children had been killed. They asked him questions that most likely will never have answers.

“What were the last moments of these people’s lives like? They were wondering, did the child even know what was happening? Were they afraid? Did they see something coming?” Weiss told NBC. “ . . . So these parents are left with those unanswered questions in addition to just why this had to happen — why to their child?’’

Elswehere in Newtown, 8-year-old Maleeha Ali, a third-grader at Sandy Hook who escaped unharmed, came with her father and mother to Treadwell Park near the school with a sign she had made honoring Vicki Soto, the first-grade teacher who reportedly died trying to protect her students from the gunman. Soto taught Maleeha when she was in first grade, and the two would exchange greetings every day, the girls’ parents said.

The sign called Soto “our hero.”

One parent who lost a child, Robbie Parker, spoke to reporters Saturday evening. He expressed sympathy for Lanza’s family, saying, “I can’t imagine how hard this experience must be for you.”

Parker said that Emilie, the daughter he lost, was blond and blue-eyed and could light up a room. “All those who had the pleasure to meet her would agree that the world was better because she was in it,” Parker said.

He recalled the last time he saw Emilie, on Friday morning as he headed to work. He had been teaching her Portuguese, and so their last conversation was in that language.

“She said that she loved me, and she gave me a kiss and I was out the door,” said Parker, whose family moved to Newtown eight months ago. “I’m so blessed to be her dad.”

Horwitz and Fahrenthold reported from Washington. Brady Dennis in Washington contributed to this report.