NEWTOWN, Conn. -- Police say 27 people died in the shooting at a Connecticut elementary school, including the gunman, and one person died at another scene.

The shooter was identified by the Associated Press as Adam Lanza, 20, who was found dead inside Sandy Hook Elementary School. At least three weapons were recovered at the scene, including a .223-caliber assault rifle from the back of a car and two semiautomatic handguns found near Lanza. His 24-year-old brother, Ryan, of Hoboken, N.J., was being questioned by authorities.

A law enforcement official -- briefed on the situation but asked not to be named because he was not authorized to speak publicly -- said that Nancy Lanza, the shooter's 52-year-old mother and was among those killed. Police say another adult male was found in a Newtown home owned by Nancy and Peter Lanza, Adam and Ryan Lanza's father.

Details about what transpired at the school and the Newton home remained sketchy several hours after the shootings, which began at about 9:30 a.m., shortly after classes began. State Police Lt. Paul Vance said authorities were still examining the gruesome crime scene and determining victims' identities and piecing together the sequence of events.

Children lucky enough to escape the carnage fled in frightened groups -- some crying, some holding hands -- as they were escorted from the single-story school by teachers. Witnesses reported up to 100 shots were fired. Vance said the victims were all shot in one section of the school, believed to be two kindergarten classrooms.

Vance said 18 of the children and six adults died at the school. Two other children were pronounced dead after they were taken to local hospitals. One wounded victim was hospitalized.

The incident -- which would be one of the worst school shootings in U.S. history -- is the latest in a series of mass shootings in the U.S. this year, including Tuesday's assault by a lone gunman at a Portland, Ore., shopping mall that left two dead and one wounded.

"It's not a simplistic scene," Vance said. " We will be here through the night, through the weekend. There is a great deal of work that has to be done." Vance later said that the murder scene was so gruesome that first responders, including tactical squad police, were provided counseling later in the day.

"This was a tragic, horrific scene they encountered,'' he said.

A visibly shaken President Obama, wiping away tears, said he was "heartbroken."

"These were "beautiful little kids between the ages of 5 and 10 years old,'' Obama said. "They had their entire lives ahead of them. Birthdays. Graduations. Weddings. Kids of their own."

Sandy Hook is in residential, wooded neighborhood about 60 miles northeast of New York City. It has has 39 teachers and about 650 students. A reverse 911 call went out to parents warning of an incident, shaking the quiet, middle and upper-middle class community of 27,000 to its core.

"This is the most tragic thing we've ever encountered,'' said Newton Police Lt. George Sinko. "We have to think about the famlies right now."

Fourth-grader Bear Nikitchyuk was heading back to his classroom when he heard someone kicking a door. "I looked behind me and all I saw was smoke and I smelled smoke. I heard shots fired. The second-grade teacher grabbed me and pulled me into her room."

The unidentified teacher locked the door and huddled about 20 kids in closets until police banged on the door. The teacher first balked at letting the police in until she was convinced they were police. The children eventually exited out the school's back door through a playground and walked to a nearby firehouse used as a staging area for fleeing kids and faculty.

Terese Lestik was relieved to find her 5-year-old daughter, Eva, unharmed.

"I heard a boom-boom,'' Eva said of the gunshots she heard earlier.

"I'm horrified,'' said Terese Lestik. "I just pray for whoever is hurt."

Alexis Wasik, a third-grader at the school, said police were checking everybody inside the school before they were escorted to the firehouse. She said she heard shots and saw her former nursery school teacher being taken out of the building on a stretcher but didn't know if the woman had been shot.

"We had to walk with a partner," said Alexis, 8. One child leaving the school said that there was shattered glass everywhere. A police officer ran into the classroom and told them to run outside and keep going until they reached the firehouse, The Hartford Courant reported.

Counselors and crisis response staff from state mental health services, children and family services, public health and the Red Cross are working with federal and local officials on the scene, Gov. Dan Malloy's office said.

Children are likely to be traumatized, says Dr. Victor Fornari, director of Child/Adolescent Psychiatry at North Shore-LIJ Health System in New Hyde Park, N.Y. Schools are supposed to be safe, nurturing environment. The shooting shatters that belief. Listening to children and trying to be supportive and reassuring can be helpful, Fornari says.

James Alan Fox of Northeastern University's School of Criminology and Criminal Justice said Friday's incident seems reminiscent of several from the late 1980s involving shooting rampages at schools.

Fox couldn't speak to the specifics of the Connecticut case, but said, "If someone is interested in punishing society where it's most vulnerable, they know that a school is a place where lots of young, innocent children, our most cherished members of society, are congregated and under their gun -- literally."

Children are often seen as "easy targets to get even with society -- or maybe it was the school. We don't know what the primary target was, and the primary motive."

Still, over the past few years, shootings in K-12 schools have become increasingly rare. After reaching a high of 63 deaths in the 2006-2007 school year, the number of people killed in "school-associated" incidents dropped to 33 last year -- lowest in two decades, according to the U.S. Department of Education.

While a few dozen children are killed each year in school, statistically speaking it remains the safest place a child will likely ever be, with the lowest chance of being killed. "When you consider the fact that there are over 50 million schoolchildren in America, the chances are over one in two million, not a high probability," said Fox. "And most cases that do occur are in high schools and less so in middle schools -- and hardly ever in elementary schools."

Police and local officials have met with family members of the victims, he said.

"It's a very difficult scene," Vance said. "It's a tragic scene. They are going through a tremendous amount of grief."

Contributing: Kevin Johnson, Greg Toppo, Liz Szabo, Donna Leinwand Leger, Associated Press

Gary Stoller and Gary Strauss, USA TODAY