The shooting occurred around 9:40 a.m. ET at Sandy Hook Elementary School in western Connecticut. Police say 18 children and six adults were declared dead at the scene. Two more children died at the hospital. One shooting victim remains hospitalized in unknown condition.
Among the adults found dead at the school were Principal Dawn Hochsprung and a school psychologist.Receive live updates from ABC News on the Connecticut school shooting
"The scene is secure. The public is not in danger," said Connecticut State Police spokesman Paul Vance.
Law enforcement officials identified the suspected gunman as Adam Lanza, age 20, who took his own life after the shooting rampage. The shooter was initially reported as his brother, Ryan Lanza, who is currently being questioned by police. Federal sources say that identification belonging to Ryan Lanza was found at the scene, which may have caused an initial misidentification. Ryan Lanza says he had not been in contact with his brother since 2010.
According to ABC News, the shooter's mother, Nancy Lanza, worked at the school as a teacher's aide. Sources say Adam Lanza shot his mother in the face at her home, and then drove to the school, where he killed 26 people.Watch Pres. Obama's national address about the Sandy Hook shooting
Sources told ABC News that the shooter was armed with four weapons, including a Sig Sauer and a Glock semi automatic handgun, and .223 shell casings, a round used in a semi automatic military style rifle, were also found. He also wore a bullet-proof vest. It was earlier reported that there might be a second gunman, but police made no mention of a second shooter at a news conference.
President Barack Obama was briefed on the shooting by FBI Director Robert Mueller. Mr. Obama ordered flags flown at half staff at all federal and military buildings and installations.
"Our hearts are broken today," Mr. Obama said as he addressed the nation. A tearful president said there's "not a parent in America who doesn't feel the overwhelming grief that I do."
Youngsters and their parents described teachers locking doors and ordering the children to huddle in the corner or hide in closets when shots echoed through the building. Police say the shooting happened in one section of the school in two rooms.
The shooting prompted the town to lock down all its schools, and SWAT teams surrounded the school. Frightened students were seen being escorted through a parking lot.
Authorities continued to question Ryan Lanza, the shooter's brother, but said he is not under any suspicion. Ryan Lanza, 24, works as a senior tax employee with Ernst & Young in Manhattan, sources told ABC News. He has worked at the firm for four years, and sources said he is "clean as a whistle."
As part of normal investigation practices, the NYPD and FBI appeared at Ernst & Young Friday to remove Ryan Lanza's laptop and other possessions. Officials are also interviewing Peter Lanza, Adam and Ryan Lanza's father, who is also not under any suspicion, multiple sources told ABC News. Numerous relatives and friends of Adam Lanza and his mother are also being interviewed by the FBI in New Jersey, Connecticut and Massachusettes, in an attempt to get a better picture of the suspected gunman and any possible motive.
This shooting is the second deadliest in U.S. history, behind the mass shooting on the campus of Virginia Tech in 2007 that left 32 people and the gunman dead.See photos from the Sandy Hook school shooting in Connecticut
The VTV Family Outreach Foundation, which consists of the families of Virginia Tech shooting victims, expressed their condolences to all affected by the shooting. They also offered their help.
"From our VTV experience, as families of the victims and survivors of the April 16, 2007 massacre at Virginia Tech, we know that their loss is incredible," Joe Samaha, President of VTV Family Outreach Foundation, said in a statement. "Helping all to deal with the aftermath will take a lifetime of love and dedication."
As a result of the shooting, police across the United States, including in Los Angeles, stepped up school security. LAPD officers provided additional patrols to city schools throughout the day, and the patrols are expected to continue when students return to school in January.
Reporter Carlos Granda is in Newtown, Conn. Look for his live reports throughout the weekend on ABC7 Eyewitness News.
ABC News and The Associated Press contributed to this report.