Connecticut State Police lead children from the Sandy Hook Elementary School in Newtown, Conn., following a shooting there, December 14, 2012.

Adam Lanza, a 20-year-old mentally ill man who reportedly had a preoccupation with mass shootings, in particular the 1999 Columbine massacre, killed his mother, Nancy Lanza, and then drove to the elementary school, where he shot and killed 26 people, including 20 first-graders, in less than 11 minutes, before taking his own life. Two other school staffers were wounded by gunfire.

It was one of the worst mass shootings in U.S. history.

A woman waits to hear about her sister, a teacher, following a shooting at the Sandy Hook Elementary School in Newtown, Conn., about 60 miles northeast of New York City, Dec. 14, 2012.

Children are seen outside following a shooting at the Sandy Hook Elementary School in Newtown, Conn., about 60 miles northeast of New York City, Dec. 14, 2012.

This satellite image provided by Google shows the Sandy Hook Elementary School in Newtown, Conn.

Connecticut State Police walk on Dickson Street from the scene of an elementary school shooting on Dec. 14, 2012, in Newtown, Conn.

A mother runs with her children as police above canvass homes in the area following a shooting at the Sandy Hook Elementary School in Newtown, Conn., about 60 miles northeast of New York City, Dec. 14, 2012.

Parents leave a staging area after being reunited with their children following a shooting at the Sandy Hook Elementary School in Newtown, Conn., Dec. 14, 2012.

A man reacts following a shooting at the Sandy Hook Elementary School in Newtown, Conn., Dec. 14, 2012.

Parents leave a staging area after being reunited with their children following a shooting at the Sandy Hook Elementary School in Newtown, Conn., Dec. 14, 2012.

A mother hugs her daughter following a shooting at the Sandy Hook Elementary School in Newtown, Conn., Dec. 14, 2012.

Law enforcement canvass the area following a shooting at the Sandy Hook Elementary School in Newtown, Conn., Dec. 14, 2012.

A man and woman leave the staging area for family around near the scene of a shooting at the Sandy Hook Elementary School in Newtown, Conn., Dec. 14, 2012.

Connecticut State Police lead children from the Sandy Hook Elementary School in Newtown, Conn., following a shooting there, Dec. 14, 2012.

Connecticut State Police walk on Dickson Street from the scene of an elementary school shooting on Dec. 14, 2012, in Newtown, Conn.

The aftermath of a shooting at Sandy Hook Elementary School in Newtown, Conn., on Dec. 14, 2012.

Law enforcement canvass an area nearby a school shooting at the Sandy Hook Elementary School in Newtown, Conn., Dec. 14, 2012.

A law enforcement agent arrives to the area following a shooting at the Sandy Hook Elementary School in Newtown, Conn., Dec. 14, 2012.

The aftermath of a shooting at Sandy Hook Elementary School in Newtown, Conn., on Dec. 14, 2012.

Connecticut State Police walk near the scene of an elementary school shooting on Dec. 14, 2012, in Newtown, Conn.

A young girl is comforted following a shooting at the Sandy Hook Elementary School in Newtown, Conn., Dec. 14, 2012.

A woman waits to hear about her sister, a teacher, following a shooting at the Sandy Hook Elementary School in Newtown, Conn., Dec. 14, 2012.

Law enforcement canvass an area following a shooting at the Sandy Hook Elementary School in Newtown, Conn., Dec. 14, 2012.

People embrace at a firehouse staging area for family around near the scene of a shooting at the Sandy Hook Elementary School in Newtown, Conn., Dec. 14, 2012.

Ambulances leave an area near the scene of a shooting at the Sandy Hook Elementary School in Newtown, Conn., Dec. 14, 2012.

A young girl is comforted following a shooting at the Sandy Hook Elementary School in Newtown, Conn., Dec. 14, 2012.

A mother hugs her daughter following a shooting at the Sandy Hook Elementary School in Newtown, Conn., Dec. 14, 2012.

Connecticut State Police walk near the scene of an elementary school shooting on Dec. 14, 2012, in Newtown, Conn.

Victims family leave a firehouse staging area following a shooting at the Sandy Hook Elementary School in Newtown, Conn., Dec. 14, 2012.

Officials are on the scene outside of Sandy Hook Elementary School in Newtown, Conn., where authorities say a gunman opened fire inside an elementary school in a shooting that left 26 people dead, including 20 children, Dec. 14, 2012.

This aerial photo shows Sandy Hook Elementary School in Newtown, Conn., right, where authorities say a gunman opened fire inside the school in a shooting that left 26 people dead, including 20 children, Dec. 14, 2012. The gunman then took his own life.

Gov. Dannel P. Malloy, right, talks with officials at a staging area following a shooting at the Sandy Hook Elementary School in Newtown, Conn., Dec. 14, 2012.

Parents leave a staging area after being reunited with their children following a shooting at the Sandy Hook Elementary School in Newtown, Conn., Dec. 14, 2012.

A mother runs with her children as police above canvass homes in the area following a shooting at the Sandy Hook Elementary School in Newtown, Conn., Dec. 14, 2012.

A woman holds a child as people line up to enter the Newtown Methodist Church near the the scene of an elementary school shooting on Dec. 14, 2012, in Newtown, Conn.

People gather outside the White House to participate in a candlelight vigil to remember the victims at the Sandy Hook Elementary School shooting in Newtown, Connecticut on Dec. 14, 2012 in Washington, D.C.

President Barack Obama, pictured in the Brady Briefing Room of the White House on Dec. 14, 2012, in Washington, D.C., following the shooting in a Connecticut elementary school in which 26 students and educators were killed.

"This evening, Michelle and I will do what I know every parent in America will do, which is hug our children a little tighter and we'll tell them that we love them, and we'll remind each other how deeply we love one another.  But there are families in Connecticut who cannot do that tonight.  And they need all of us right now.  In the hard days to come, that community needs us to be at our best as Americans."

A woman comforts a young girl during a vigil service for victims of the Sandy Hook Elementary shooting, Friday, Dec. 14, 2012, at St. Rose of Lima Roman Catholic Church in Newtown, Conn.

Men and women hold candles in vigil outside St. Rose of Lima Roman Catholic Church during a healing service held in for victims of an elementary school shooting in Newtown, Conn., Friday, Dec. 14, 2012. 

Connecticut Gov. Dannel Malloy, left, speaks to mourners at a vigil service for victims of the Sandy Hook Elementary School shooting, at the St. Rose of Lima Roman Catholic Church in Newtown, Conn. Friday, Dec. 14, 2012. 

A horsedrawn carriage carrying the body of Ana Grace Márquez-Greene - one of the 20 students killed at Sandy Hook -  leaves the church after her funeral in Bloomfield, Conn., Saturday, Dec. 22, 2012. 

Firearms training unit Detective Barbara J. Mattson, of the Connecticut State Police, holds up a Bushmaster AR-15 rifle, the same make and model of gun used by Adam Lanza in the Sandy Hook School shooting, for a demonstration during a hearing of a legislative subcommittee reviewing gun laws, at the Legislative Office Building in Hartford, Conn., Jan. 28, 2013.

In this crime scene evidence photo provided by the Connecticut State Police, firearms and ammunition found on or in close proximity to the body of the gunman at Sandy Hook Elementary School following the December 14, 2012 shooting rampage, is shown.

In Adam Lanza's home, police found a spreadsheet of mass murders, photographs of Lanza holding a gun to his head, multiple firearms, and "many edged weapons, knives, swords, spears, etc."

Sandy Hook Elementary students are pictured on a school bus in Newtown, Conn., on Jan. 3, 2013. Students at the elementary school, where a gunman killed 20 students and six educators on Dec. 14, 2012, restarted their academic year by returning to class at an alternative campus in the nearby town of Monroe, described by police as "the safest school in America." 

A sign welcoming children from Sandy Hook Elementary school sits on the road in Monroe, Conn., on Jan. 3, 2013.

Roses with the faces of the Sandy Hook Elementary students and adults killed are seen on a pole in Newtown, Conn., on Jan. 3, 2013.

In a November 2013 report, Connecticut State Attorney Stephen J. Sedensky III wrote, "The obvious question that remains is: Why did the shooter murder twenty-seven people, including twenty children? Unfortunately, that question may never be answered conclusively… The shooter planned his actions, including the taking of his own life, but there is no clear indication why he did so, or why he targeted Sandy Hook Elementary School.

"It is known that the shooter had significant mental health issues that affected his ability to live a normal life and to interact with others, even those to whom he should have been close."

Some of the remaining memorial items to Sandy Hook Elementary students and staff who died are viewed in Newtown, Conn., on Jan. 3, 2013.

Dave Kleiner, of Gettysburg, Pa., sells antique guns at a gun show at the Crowne Plaza Hotel on Jan. 5, 2013, in Stamford, Conn., 30 miles from Newtown. While other area gun shows were canceled following the Sandy Hook rampage, a gun show held by Westchester Collectors Inc. in Stamford proceeded as planned.

In the wake of the shooting, the National Rifle Association pushed back against stronger gun laws or limits on ammunition. NRA President Wayne LaPierre instead proposed putting armed guards in every school in America. "The only thing that stops a bad guy with a gun is a good guy with a gun," he said.

Nelba Marquez Greene, left, and Nicole Hockley, both mothers of Sandy Hook Elementary massacre victims, console each other during a press conference on the one-month anniversary of the Newtown elementary school massacre on Jan. 14, 2013, in Newtown, Conn.

Parents of Sandy Hook Elementary School massacre victims speak during a press conference on Jan. 14, 2013, in Newtown, Conn. Eleven families of Sandy Hook victims came to the event one month after the shooting to give their support to Sandy Hook Promise, co-founded by Makris, a new non-profit with the goal of preventing such tragedies in the future.

Jackie Barden, whose son, Daniel, was killed in the Sandy Hook Elementary massacre, kisses her 10-year-old daughter, Natalie, during a press conference on the one-month anniversary of the Newtown elementary school massacre on Jan. 14, 2013, in Newtown, Conn.

Community members embrace at a press conference with parents of Sandy Hook Elementary victims on the one-month anniversary of the Newtown elementary school massacre on Jan. 14, 2013, in Newtown, Conn.

Nelba Márquez-Greene, whose six-year-old daughter, Ana Grace Márquez-Greene, was killed in the Sandy Hook massacre, is comforted during a press conference on the one-month anniversary of the Newtown elementary school massacre on Jan. 14, 2013, in Newtown, Conn.

Ian Hockley embraces a photo of his son, Dylan Hockley, 6, during a press conference with fellow parents of victims on the one-month anniversary of the Newtown elementary school massacre on Jan. 14, 2013, in Newtown, Conn.

A small memorial of candy canes lie on the grass near Sandy Hook Elementary School on Jan. 14, 2013, in Newtown, Conn.

Neil Heslin holds a picture of himself with his six-year-old son Jesse, who was slain in the Sandy Hook massacre in Newtown, Conn., while testifying before the Senate Judiciary Committee on the Assault Weapons Ban of 2013, on Capitol Hill in Washington, D.C., Feb. 27, 2013. The following month, the legislation was tabled when it became clear that it could not get past a GOP filibuster.

Concerned citizens donated stuffed animals, toys, school supplies, bikes, and other items to the children of Newtown. "I think it was a nice gesture," said Chris Kelsey, who worked for Newtown at the time. "There was a need to do something for the kids. There was a need to make people feel better."

The town had to get a warehouse to hold the 67,000 teddy bears. "I think a lot of the stuff that came into the warehouse was more for the people that sent it, than it was for the people in Newtown," Kelsey told CBS News.

Every child in Newtown got a few bears from the overwhelming response; the rest was sent away, along with the bikes and blankets.

Connecticut Gov. Dannel P. Malloy signs legislation at the Capitol in Hartford, Conn., April 4, 2013, that includes new restrictions on weapons and large-capacity ammunition magazines, a response to the deadly school shooting in Newtown. Neil Heslin (behind left), father of Sandy Hook shooting victim Jesse Lewis; Nicole Hockley, right, mother of Sandy Hook School shooting victim Dylan; and Conn. Lt. Gov. Nancy Wyman, left, look on.

Ten months after the shooting, the Sandy Hook Elementary School was demolished, piece by piece. Contractors were asked to destroy materials to eliminate nearly every trace of the building. 

A new school

In this Dec. 1, 2015 photo, slips of paper listing acts of kindness adorn the wall of the Pleasant Valley Elementary School in South Windsor, Conn. The school is one of many across the country which asked children to perform 26 acts of kindness on the third anniversary of the shooting at the Sandy Hook Elementary school that took 26 lives. The children at Pleasant Valley - many of whom are too young to remember the shooting - were not told about the origin of the program.

Sandy Hook Promise, an organization formed by the parents of victims, has trained more than a million students and teachers in how to recognize warning signs of a troubled youth. In one video, amid the bustle of life at one school, a troubled young man is seen in the background going down a dangerous path.

"Sandy Hook was preventable," Sandy Hook Promise co-founder Nicole Hockley told CBS' "60 Minutes." "And had someone been able to see those signs and signals that our shooter gave off throughout his life, and connect those dots, and make an intervention, I wouldn't be sitting here talking to you today."  

The new 86,000-square-foot Sandy Hook Elementary School opened in 2016, featuring three courtyards, study spaces designed to look like treehouses, and a moat-like raingarden - but no obvious memorials to the 26 people who died in December 2012.

The school was designed, officials said, to be attractive, environmentally friendly, conducive to learning and, above all, safe. Its ground floor is elevated, making it harder to see inside classrooms from the outside, and all the doors and windows are bulletproof.