NEWTOWN, Conn. - Twenty school-children were slaughtered by a heavily armed gunman who opened fire at a suburban elementary school in Connecticut on Friday, ultimately killing at least 27 people including himself in the one of the worst mass shootings in US history.The 20-year-old gunman, identified by law enforcement sources as Adam Lanza, fired what witnesses described as dozens of shots at Sandy Hook Elementary School in Newtown, Connecticut, which serves children from ages 5 to 10.'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.Dozens of shots firedThe 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.Melissa Murphy, who lives near the school, monitored events on a police scanner."I kept hearing them call for the mass casualty kit and scream, 'Send everybody! Send everybody!'" she said. "It doesn't seem like it can be really happening. I feel like I'm in shock."A girl described to NBC Connecticut hearing seven loud "booms" while she was in gym class. Other children began crying and teachers moved the students to an office, she said."A police officer came in and told us to run outside and so we did," the unidentified girl said on camera.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.Obama vows 'meaningful action'Obama ordered flags flown at half-staff at US public buildings."As a country, we have been through this too many times," Obama said, ticking off a list of recent shootings."We're going to have to come together and take meaningful action to prevent more tragedies like this, regardless of the politics," Obama said in apparent reference to the influence of the National Rifle Association over members of Congress.Obama remains committed to trying to renew a ban on assault weapons, White House spokesman Jay Carney said.New York Mayor Michael Bloomberg, founder of the advocacy group Mayors Against Illegal Guns, said it was "almost impossible to believe that a mass shooting in a kindergarten class could happen."We need immediate action. We have heard all the rhetoric before. What we have not seen is leadership - not from the White House and not from Congress," he said. "That must end today."Outside the White House gates, about 200 people rallied on a cold evening in favor of gun restrictions.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.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."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.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."(The) important thing is that we're here for the families, and it won't be just tonight. It will be as long as is necessary for them to grieve, for them to come out of their grievance and come back to normal, although I don't see how you can actually come back to normal after something like this," said Kenneth Adams, 81, as he entered the service with his wife, Amelia.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.