NEWTOWN, Connecticut - Connecticut State Police believe they have the information that will help them determine how and why a man went on a killing spree at Sandy Hook Elementary School in Newtown, Connecticut on Friday.
Investigators said they found useful evidence at both the school and the suspect's home.
"Our investigators at the crime scene... did produce some very good evidence in this investigation that our investigators will be able to use in hopefully painting the complete picture as to how this and more importantly, why this occurred," said Lt. Paul Vance, Connecticut State Police.
The gunman has been identified by the Associated Press and other media agencies as Adam Lanza, 20. It's believed Lanza shot and killed 20 children and six adults at the school before taking his own life.
Vance said investigators have determined where the gunman got into the school.
"It's believed he was not voluntarily let into the school at all," Vance said. "That he forced his way into the school."
There are numerous broken windows around the school, but Vance said many of those were broken by law enforcement teams that forced their way into the school to try and save lives.
Only one shooting victim survived
Vance said one woman was shot and survived.
"She is doing fine," Vance said. "She will be instrumental in this investigation."
Initial reports said the woman was a teacher who was shot in the foot, however, that has not been confirmed by officials.
Gunman identified as Adam Lanza
A law enforcement official said Adam Lanza had a possible personality disorder and ABC News reported that the 20-year-old lived with his mother, Nancy Lanza, who had described her younger son as "troubled." Friends told ABC News that Adam Lanza was introverted and had a hard time connecting with people, but was smart. The friend said Adam went out of his way not to attract attention.
The older brother, Ryan Lanza, has been extremely cooperative, law enforcement officials said. The AP said he was not believed to have any involvement in the rampage and was not under arrest or in custody, but investigators were still searching his computers and phone records.
Ryan Lanza told law enforcement he had not been in touch with his brother since about 2010.
ABC News reported that after Lanza shot and killed his mother at home, he drove to the school in his mother's car. Three guns were found - a Glock, a Sig Sauer and a .223-caliber Bushmaster assault rifle.
ABC News reported that the guns were purchased legally by Lanza's mother, Nancy.
Teachers hailed as heroes
Authorities didn't say exactly how the shootings unfolded.
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.
First grade teacher Kaitlin Roig, 29, locked her 14 students in a class bathroom and listened to "tons of shooting" until police came to help, ABC News reported.
"It was horrific," Roig said. "I thought we were going to die."
She said that the terrified kids were saying, "I just want Christmas…I don't want to die. I just want to have Christmas."
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 told the AP. "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.
Richard Wilford's 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."
Theodore Varga said he was in a meeting with other fourth-grade teachers when he heard the gunfire, but there was no lock on the door.
He said someone turned on the public address system so that "you could hear the hysteria that was going on. I think whoever did that saved a lot of people. Everyone in the school was listening to the terror that was transpiring."
Also, a custodian went running around, warning people there was a gunman in the school, Varga said.
"He said, 'Guys! Get down! Hide!'" Varga said. "So he was actually a hero." The teacher said he did not know if the custodian survived.
Varga said he tried to kick out an air conditioning unit in the window so the five teachers in the room could escape, but he only managed to knock out the wood next to it, and the space wasn't big enough for all of them to squeeze through.
He said he smelled gun smoke in the halls as he ran out to escape through a door. Varga then went around to help three other teachers climb out of the window of the first-floor room they had been in.
Sandy Hook Elementary School had new security system
When police arrived to secure the situation, the school kids were told to close their eyes by police as they were led from the building. Police didn't want the kids to see the carnage.
Schoolchildren - some crying, others looking frightened - were escorted through a parking lot in a line, hands on each other's little shoulders.
Among the dead are school principal Dawn Hochsprung, who recently installed a new security system at Sandy Hook Elementary School.
In a letter to parents this fall, she said every visitor would be required to ring a doorbell at the school's front entrance after the doors locked at 9:30 a.m. Visitors would be required to go to the office and sign in with photo identification.
President, nation mourns
"Our hearts are broken today," a tearful President Barack Obama, struggling to maintain composure, said at the White House. He broke down, talking about the victims of the shootings.
"The majority of those who died were children -- beautiful little kids between the ages of 5 and 10 years old ... 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," Obama said.
Obama's comments on the tragedy amounted to one of the most outwardly emotional moments of his presidency. 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.
He called for "meaningful action" to prevent such shootings and ordered flags flown at half-staff until Dec. 18.
"Evil visited this community today and it's too early to speak of recovery, but each parent, each sibling, each member of the family has to understand that Connecticut -- we're all in this together. We'll do whatever we can to overcome this event," Gov. Dannel Malloy said.
The rampage, 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.
For those in Colorado, 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."
Newtown is a prosperous community of about 27,000 people 60 miles northeast of New York City.
Adam Lanza and his mother lived in a well-to-do part of Newtown where neighbors are doctors or hold white-collar positions at companies such as General Electric, Pepsi and IBM