NEWTOWN, CT - A man opened fire Friday inside the Connecticut elementary school where his mother worked as a teacher, killing 26 people, including 20 children.
The killer, armed with two handguns, took his own life and another person was found dead at a second scene, bringing the toll to 28, authorities said.
CNN says Investigators have been told that Connecticut school shooter Adam Lanza had an altercation with four adults at the school on Thursday, according to a law enforcement source with knowledge of the investigation. Three of those adults, the source said, were killed during Friday's shooting.
The attack, coming less than two weeks before Christmas, was the nation's second-deadliest school shooting, exceeded only by the Virginia Tech massacre that left 33 people dead in 2007.
Panicked parents raced to Sandy Hook Elementary School, about 60 miles northeast of New York City, looking for their children. Students were told to close their eyes by police as they were led from the building.
Schoolchildren -- some crying, others looking frightened -- were escorted through a parking lot in a line, hands on each other's shoulders.
"Our hearts are broken today," a tearful President Barack Obama, struggling to maintain composure, said at the White House. He called for "meaningful action" to prevent such shootings.
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. Authorities said the shootings took place in two rooms, but they gave no details on exactly how they unfolded.
A law enforcement official identified the gunman as 20-year-old Adam Lanza, the son of a teacher. A second law enforcement official said his mother, Nancy Lanza, was presumed dead.
The Associated Press says Lanza killed his mother at their home, and then went on the rampage at the school.
The gunman was believed to suffer from a personality disorder and lived with his mother, said a law enforcement official who was briefed on the investigation but was not authorized to discuss it.
Adam Lanza's older brother, 24-year-old Ryan, of Hoboken, N.J., was being questioned, the first official said.
The suspect was first identified as Ryan Lanza by several news sources, including ABC News, CNN, and The Associated Press.
After reports of confusion and conflicting reports, the suspect was later identified as Adam Lanza, who was Ryan's brother.
Adam Lanza died at the scene, The Associated Press reported.
A law enforcement official briefed on the investigation said the brother of the gunman has been "extremely cooperative" and is not under arrest.
The official told The Associated Press that Ryan Lanza, of Hoboken, N.J., is still being questioned but is not in custody and is not believed to have any connection to the school killings.
The official spoke on condition of anonymity because he was not authorized to speak publicly on the investigation.
The official said Ryan Lanza's computers and phone records were being searched but only "in an abundance of caution."
He said Ryan told authorities he had not been in touch with his brother in recent years.
Lanza's mother was among the dead Friday morning at Sandy Hook Elementary School in Newtown, Connecticut, according to ABC News. She was a teacher at the school.
A law enforcement official said Lanza's girlfriend and another friend are missing in New Jersey.
Both officials spoke on the condition of anonymity because they were not authorized to speak on the record about the unfolding investigation.
The gunman drove to the school in his mother's car, the second official said. Three guns were found -- a Glock and a Sig Sauer, both pistols, inside the school, and a .223-caliber rifle in the back of a car.
Lanza's girlfriend and another friend were missing in New Jersey, the official also said.
Robert Licata said his 6-year-old son was in class when the gunman burst in and shot the teacher.
"That's when my son grabbed a bunch of his friends and ran out the door," he said. "He was very brave. He waited for his friends."
He said the shooter didn't utter a word.
Stephen Delgiadice said his 8-year-old daughter was in the school and heard two big bangs. Teachers told her to get in a corner, he said.
"It's alarming, especially in Newtown, Connecticut, which we always thought was the safest place in America," he said. His daughter was fine.
Mergim Bajraliu, 17, heard the gunshots echo from his home and ran to check on his 9-year-old sister at the school. He said his sister, who was fine, heard a scream come over the intercom at one point. He said teachers were shaking and crying as they came out of the building.
"Everyone was just traumatized," he said.
Mary Pendergast, who lives close to the school, said her 9-year-old nephew was in the school at the time of the shooting, but wasn't hurt after his music teacher helped him take cover in a closet.
7-year-old son, Richie, is in the second grade at the school. His son told him that he heard a noise that "sounded like what he described as cans falling."
The boy told him a teacher went out to check on the noise, came back in, locked the door and had the kids huddle up in the corner until police arrived.
"There's no words," Wilford said. "It's sheer terror, a sense of imminent danger, to get to your child and be there to protect him."
On Friday afternoon, family members were led away from a firehouse that was being used as a staging area, some of them weeping. One man, wearing only a T-shirt without a jacket, put his arms around a woman as they walked down the middle of the street, oblivious to everything around them.
Another woman with tears rolling down her face walked by carrying a car seat with a young infant inside and a bag that appeared to have toys and stuffed animals.
The shootings instantly brought to mind episodes such as the Columbine High School massacre that killed 15 in 1999 and the July shootings at a movie theater in Aurora, Colo., that left 12 dead.
"You go to a movie theater in Aurora and all of a sudden your life is taken," Columbine principal Frank DeAngelis said. "You're at a shopping mall in Portland, Ore., and your life is taken. This morning, when parents kissed their kids goodbye knowing that they are going to be home to celebrate the holiday season coming up, you don't expect this to happen. I think as a society, we need to come together. It has to stop, these senseless deaths."
Obama's comments on the tragedy amounted to one of the most outwardly emotional moments of his presidency.
"The majority of those who died were children -- beautiful, little kids between the ages of 5 and 10 years old," Obama said.
He paused for several seconds to keep his composure as he teared up and wiped an eye. Nearby, two aides cried and held hands as they listened to Obama.
"They had their entire lives ahead of them -- birthdays, graduations, wedding, kids of their own," Obama continued about the victims. "Among the fallen were also teachers, men and women who devoted their lives to helping our children."