Newtown - This time, the shooter killed 27 people, including his elementary-schoolteacher mother, her principal and 20 children, most of them kindergartners. Then the shooter killed himself.

Friday morning's massacre in Newtown - the second-deadliest shooting event in U.S. history - took place at Sandy Hook Elementary School, where more than 600 children from kindergarten through fourth grade spend their days. About 9:30 a.m., the day was interrupted by a burst of sound on the school intercom: screams.

That was enough for some teachers to move their students away from doors and windows. In one room, a teacher locked her students in a closet, a move parents said saved their children's lives. In another class, a teacher heard two blasts and ordered students into a corner.

Jessica Eisele, a fourth-grader at Sandy Hook, was in the gym when the shooting began. "On the loudspeaker, there was screaming and crying and she heard gunshots and then silence," said her brother Nick, 15.

A janitor ran through the halls, shouting that a gunman was in the building. Witnesses heard dozens of pops coming from two classrooms and a hallway.

In the main office, the principal, vice principal and school psychologist heard the noise and stepped into the hall to investigate. A witness told CNN that the vice principal crawled back into the office after being shot in the foot. The principal was later found dead.

Around Newtown, word spread by text, phone and the sound of dozens of emergency vehicles thundering toward the school. At Berkshire Motors, owner Jim Marcucilli thought he saw 40 police cruisers speed by. He had been working on a car, but its owner, frantic, demanded her keys back. "Oh, my God," she said, "I have to get my kid."

Joseph Wasik, 42, wasn't too worried when his wife, Lynn, called to say she had received a text alert from the school. Like schools nationwide, Sandy Hook had been on lockdown before, always for events that turned out to be nothing. Earlier this year, reports of a loose bear seen in town had activated the system.

But when Wasik flipped open his laptop and saw a report of a school shooting, he got in his car and headed to Sandy Hook, where his daughter Alexis is a third-grader. "I flew down there," said Wasik, an electrician.

When he arrived, the backup of emergency vehicles and panicked parents extended half a mile beyond the school. Wasik parked on the first lawn he could find and ran.

"It was chaos, cars blocking everything, a SWAT team," he said. "All these parents screaming for their kids."

Children were ushered out of the building, instructed to close their eyes as adults guided them through the halls. Some were so scared, they vomited.

The students were moved into the parking lot, single file, hands on one another's shoulders. Some wept, some smiled, some stared at the pavement.

A few of the children had blood on their clothes.

Helicopters circled above and dozens of emergency vehicles arrived from surrounding towns. Firefighters rushed over to parents holding their children and wrapped them in blankets to warm them from the December chill.

Stephen Delgiadice's 8-year-old daughter cowered with her classmates in a corner of their room until police led them out. "It's alarming, especially in Newtown, Connecticut, which we always thought was the safest place in America," he told the Associated Press. His daughter was unharmed.

Law enforcement sources identified the shooter as Adam Lanza, 20, the son of Nancy Lanza, the kindergarten teacher whose body was found at the home she shared with Adam.

Authorities earlier Friday had misidentified the gunman as Ryan Lanza, Adam's older brother. Ryan Lanza was questioned by police, who concluded that he had no connection to the massacre. Ryan said he had not been in contact with his brother for more than two years.

Reached at her home in Florida, Nancy Lanza's mother said she could not fathom the violence that took her daughter's life. "I just can't cope with it right now," Dorothy Hanson said through tears. "She was my daughter and a beautiful girl and I loved her. I just can't make any more comments than that right now."

Adam Lanza, who lived with his mother, was unusually prone to tantrums as a child and took medication for behavioral issues, according to friends and neighbors of the family. Nancy and Peter Lanza separated about 10 years ago. Nancy remained in Newtown, and Peter, who remarried, lives in Stamford, friends said.

Once again, flags were lowered to half-mast across the land. In a television address, President Barack Obama said that "As a nation, we have been through this too many times." He broke off and took a long breath to suppress a sob. He wiped his eyes and pushed ahead. "These children are our children."

Along with many other politicians, he pledged "meaningful action to prevent more tragedies like this," but did not offer specifics. White House spokesman Jay Carney said Friday was a day "to feel enormous sympathy for families that are affected and to do everything we can to support state and local law enforcement. ... I'm sure there will be a day for discussion of the usual Washington policy debates, but I don't think today is that day."