At least 28 people, including children, are reported to have been killed after at least one shooter opened fire at a school in Newtown, Connecticut.
Reports say 20 people killed in the Sandy Hook Elementary School on December 14 were children in what is seen as one of the worst school shootings in American history.
Police said the suspect was 20-year-old Adam Lanza, who is also reported to have died in the incident.
There are reports that he killed his mother at their nearby home before going to the school. It was unclear whether the mother worked at the school as had first been reported.
Lieutenant Paul Vance from the Newtown police told CNN that the school is now secure.
"Newtown police immediately upon arrival entered the school and began a complete active shooter’s search of the building," he said. "That included checking every door, every portion of that school. Our main objective was to evacuate as quickly and efficiently as possible any and all students and faculty in the school."
The Sandy Hook Elementary School has children from kindergarten through fourth grade aged between 5 and 10 years.
One Newtown resident told reporters that "a masked man" entered the school "and just started shooting."
"I don’t know how he came in because every time you come in you have to ring a bell, and they can see you from the [school] office and they know who you are, and they let you in,” she added.
Speaking in response to the tragedy, a visibly moved President Barack Obama said the nation's "heart was broken."
"The majority of those who died today were children, beautiful little kids between the ages of 5 and 10 years," he said in a TV address. "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. So our hearts are broken today."
The attack at Sandy Hook Elementary School was the latest of several mass shootings in the U.S. this year and one of the worst such incidents ever.
Obama maintained that the United States has suffered too many such attacks.
"As a country, we have been through this too many times, whether it's an elementary school in Newtown [Connecticut] or a shopping mall in Oregon or a temple in Wisconsin or a movie theater in Aurora [Colorado] or a street corner in Chicago," he said. "These neighborhoods are our neighborhoods and these children are our children and we're going to have to come together to take meaningful action to prevent more tragedies like this, regardless of the politics."
The United States is observing four days of official mourning, which began on December 14.
World leaders have expressed shock and sorrow at the tragedy.
British Prime Minister David Cameron said he was "deeply saddened," by the killings.
Japanese Prime Minister Yoshihiko Noda offered his "condolences," while Australian Prime Minister Julia Gillard described the attack as "evil."
Russian President Vladimir Putin, Afghan President Hamid Karzai, Pope Benedict XVI, French President Francois Hollande, and UN Secretary-General Ban Ki-moon also sent sympathy.
Iranian foreign ministry spokesman Ramin Mehmanparast said the killings were "tragic."
Earlier this year, a gunman killed 12 people at a cinema in Colorado. In Wisconsin, another gunman killed six people before shooting himself at a Sikh temple.
With reporting by Reuters, AP, AFP, and dpa