A gunman is believed to have slain his mother at their home before unleashing a hail of gunfire that killed 20 children and six adults Friday in the Connecticut elementary school where she was a teacher, dpa reported.
Police said the suspect was dead. Media reports cited law enforcement sources as saying that Adam Lanza, 20, apparently took his own life inside Sandy Hook Elementary School after the shooting spree.
The mass shooting shook Newtown, a quiet, affluent suburb of 27,500 people about 100 kilometres north-east of New York City.
"Our hearts are broken today - for the parents and grandparents, sisters and brothers of these little children and for the families of the adults who were lost," said President Barack Obama, wiping away tears and pausing a number of times to compose himself while speaking to reporters at the White House.
"They had their entire lives ahead of them - birthdays, graduations, weddings, kids of their own. Among the fallen were also teachers - men and women who devoted their lives to helping our children fulfill their dreams," he said.
Obama recalled a shopping mall shooting Tuesday in Oregon state and killing sprees in July at a Colorado cinema and in August at a Sikh temple in Wisconsin: "As a country, we have been through this too many times."
Police in Newtown said 18 children died at the scene and two others died of their wounds at a nearby hospital.
The gunman's mother was a kindergarten teacher, and many of the children killed were in her classroom, news reports quoted law enforcement officials as saying. Among the dead were the principal and a school psychologist.
More than 600 pupils from kindergarten to fourth grade, aged 5 to 10, attend the school.
By late Friday, widespread reports said an adult found in a home in Newtown away from the school was the suspected killer's mother.
Motives in the killing spree were unclear.
Police rushed to the school after receiving an emergency call just after 9:30 am (1430 GMT).
News reports described scenes of children hiding in closets when they heard the gunfire.
"Then we heard screaming," a student told television station WABC. "We went to the wall, and we sat down, and then police came in and said, 'Is he in here?' Then somebody yelled, 'Get to a safe place,' so we went to a closet in the gym, and we sat there for a little while."
The pupils were able to run out of the school when police knocked on the door and announced that evacuation was under way.
Others were instructed by their teacher to hide in the closet, Brenda Lebinski, mother of an 8-year-old girl, told the Newtown Patch website.
Lebinski said she rushed to the school after receiving an automated notification that the building was locked down. She saw hundreds of parents waiting there, and some told her that bloodied children were being carried out.
On spotting her daughter, Lebinski said she grabbed the child in "the happiest moment of my entire life."
"My daughter's teacher is my hero," Lebinski said. "She locked all the kids in a closet, and that saved their lives."
In Washington, gun control activists gathered at dusk outside the White House to demand tougher firearms laws. About 100 people rallied on Pennsylvania Avenue, singing during a solemn, candlelight vigil.
In prayer service held Friday night at Newtown's Roman Catholic Church, Connecticut Governor Dan Malloy said, "Evil visited this community today."
"May God bless our children who are with us today and those who were taken away," he said while also offering prayers for the slain adults.