Prime Minister Binyamin Netanyahu on Saturday sent a letter to US President Barack Obama expressing "profound grief" over the mass shooting at an elementary school in Newtown, Connecticut.

Twenty school-children on Friday were slaughtered by heavily armed gunman Adam Lanza, 20, who ultimately killed 28 people including himself in one of the worst mass shootings in US history.

"I was shocked and horrified by [the] savage massacre of innocent children and adults at the Sandy Hook Elementary School in Connecticut," Netanyahu wrote.

"We in Israel have experienced such cruel acts of slaughter and we know the shock and agony they bring," he continued. "I want to express my profound grief, and that of all the people of Israel, to the families that lost their loved ones."

The prime minister concluded by conveying his "deepest condolences" and wishes for "the American people [to] find the strength to overcome this unspeakable tragedy."

President Shimon Peres likewise on Saturday sent a letter of support to his US counterpart, saying that "as friends and parents" the Israeli nation shares the US' deep sorrow over the terrible and incomprehensible murder.

"There is no crime more shocking than the murder of children," Peres continued, stressing that the "collective heart of the people of Israel" is with the families of the victims, with the grieving community of Newton and with the American people as a whole.

"Our thoughts and prayers are with you," Peres concluded.

Earlier Saturday, Tzipi Livni sent US Secretary of State Hillary Clinton a letter of condolence.

"Senseless violence has shown its ugliest face, since it struck at civilized society's most vulnerable spot, our young children. The human soul refuses to comprehend these images of bloodshed and grief," The Tzipi Livni Party leader wrote.

"On this fateful day when tragedy struck at the good people of Newtown, I grieve, along with my fellow Israelis, the loss of so many lives of innocent men, women and children. Our hearts go out to the families of the victims," she added.

A shocked community begins to mourn

Authorities found 18 children and seven adults, including the gunman, dead at the school, and two children were pronounced dead later after being taken to a hospital. Another adult was found dead at a related crime scene in Newtown, bringing the toll to 28, state police Lieutenant Paul Vance said.

As reports of the shooting spread, panicked parents rushed to the school searching for their children as students covered in blood were being carried out of the building.

US President Barack Obama, wiping away tears and pausing to collect his emotions in an address to the nation, mourned the "beautiful little kids between the ages of 5 and 10 years old" who were killed.

"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," Obama said, his voice cracking.

Hundreds of Newtown residents gathered to mourn on Friday night at St. Rose of Lima, the Catholic church just a couple of miles (km) from the school.

Connecticut Governor Dannel Malloy and Senator Richard Blumenthal spoke at the service, although the crowd appeared most moved by Monsignor Robert Weiss, who had spent the day at a firehouse consoling victims' families.

"Life has changed forever in Newtown," Weiss said. "We have 20 new saints today. We have 20 beautiful angels."

People lingered well after the service had ended. A middle-aged man sat with his face in his hands, elbows on a pew. A woman behind him prayed with her hands together, fingertips to her lips. Nearby, a man in work boots hugged a friend.

The holiday season tragedy was the second shooting rampage in the United States this week and the latest in a series of mass killings this year, and was certain to revive a debate about US gun laws.

'Always very concerned about her son'

Early media reports suggested that Lanza's mother, Nancy, was a teacher at the school and that he shot her and her students. But by evening, many media accounts indicated that Lanza's mother was the fatality at the second crime scene. Her connection to the school was unclear.

Nancy Lanza was "very nice, very pleasant and always very appreciative of our work," said Dan Holmes, owner of Holmes Fine Gardens, a landscaping firm in Newtown.

Holmes, who last week decorated her yard with Christmas garlands and lights, said Nancy Lanza was an avid gun collector who once showed him a "really nice, high-end rifle" she had purchased.

"She said she would often go target shooting with her kids," said Holmes. "She was always very concerned about her son."

State police refused to confirm any details about the Lanzas, saying they hoped to have more information on Saturday.

The New York Times reported Adam Lanza used a Sig Sauer and a Glock, both handguns, and said police also found at the scene a Bushmaster .223 M4 carbine, a rifle, that they believe belonged to him.

His brother, Ryan Lanza, was "either in custody or being questioned," a law enforcement source said.

Panic breaks out after dozens of shots fired

The chaos struck as children gathered in their classrooms for morning sessions at Sandy Hook Elementary in Newtown, a wealthy, wooded suburb of 27,000 in Fairfield County, about 80 miles (130 km) northeast of New York City.

A state police spokesman said the shootings took place in two rooms. Witnesses reported hearing dozens of shots; some said as many as 100 rounds.

Images from the scene showed children being led away in single file, each child's hands clutching the shoulders of the one in front. Police wearing body armor and carrying rifles swarmed the scene and locked down the school.

Nearly 12 hours later, the bodies of the dead children, adults and gunman remained in the school awaiting identification.

The toll in Newtown exceeded that of one of the most notorious US school shootings, the 1999 rampage at Columbine High School in Littleton, Colorado, where two teenagers killed 13 students and staff before killing themselves.

The United States has seen a number of shooting rampages this year, most recently in Oregon, where a gunman killed two people and then himself at a shopping mall on Tuesday. The deadliest came in July at a midnight screening of a Batman film in Colorado that killed 12 people and wounded 58.

In 2007, 32 people were killed at Virginia Tech university in the deadliest act of criminal gun violence in US history.

International community expresses support for US

French President Francois Hollande, in an open letter to Obama, said he was "horrified" by the shootings.

British Prime Minister David Cameron said, "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."

UN Secretary General Ban Ki-moon expressed his condolences and called the targeting of children "heinous and unthinkable."

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