Lanza attended Sandy Hook Elementary School for four and a half years. He began at Newtown Middle School in 2004, but according to his mother was “wracked by anxiety”. She told friends that her son started getting upset at middle school because of frequent classroom changes during the day. The movement and noise was too stimulating and made him anxious. At one point his anxiety was so intense that she took him to the emergency room at Danbury Hospital. In April 2005, she moved him to a new school, St. Rose of Lima, where he lasted only eight weeks.

At age 14, he went to Newtown High School, where he was named to the honor roll in 2007. Students and teachers who knew him in high school described Lanza as “intelligent but nervous and fidgety”. He avoided attracting attention and was uncomfortable socializing. He is not known to have had any close friends in school. Schoolwork often triggered his underlying sense of hopelessness and by 2008, when he turned 16, he was only going to school occasionally. The intense anxiety Lanza experienced at the time suggests his autism was exacerbated by the hormonal shifts of adolescence. He was taken out of high school and home-schooled by his mother and father. He earned a GED. In 2008 and 2009, he also attended some classes at Western Connecticut State University.

On December 21, 2012, the National Rifle Association’s Wayne LaPierre said gun-free school zones attract killers and that another gun ban would not protect Americans. He called on Congress to appropriate funds to hire armed police officers for every American school and announced that the NRA would create the National School Shield Emergency Response Program to help. After LaPierre’s press conference, the Brady Campaign asked for donations to support its gun control advocacy and asked NRA members “who believe like we do, that we are better than this” to join its campaign. On January 8, 2013, former Congresswoman Gabrielle Giffords, who was shot and injured in a 2011 shooting in Tucson, launched the gun control group Americans for Responsible Solutions, with a specific aim of matching or exceeding the fundraising capabilities of the NRA and similar groups.

A few days later, the Newtown-Sandy Hook Community Foundation released results of a survey with over 1,600 respondents. Among other inquiries, the survey asked residents what should be done with balance of the US$11 million in donations that had been received since the incident in 2012. The majority of responses said that money for mental health counseling and other family expenses should be the top priorities. A few responses suggested that some of the money should be used to purchase and tear down the shooter’s family home in order to replace it with a park or wildlife sanctuary. Jennifer Barahona, the foundation’s executive director, was quoted as saying, “That’s not something we’re considering at this time. It’s really outside of our scope.”

On December 4, 2013, seven 911 calls relating to the shooting were made public.

On March 28, 2013, court documents released from the investigation showed that the school shooting had occurred in the space of less than five minutes with 156 shots fired. This comprised 154 shots from the rifle and two shots from the 10mm pistol. Lanza fired one shot from the Glock in the hallway and killed himself with another shot from the pistol to the head.

Investigators evaluated Lanza’s body, looking for evidence of drugs or medication through toxicology tests. Unusually for an investigation of this type, DNA testing of Lanza was utilized. The results of the toxicology report were published in October 2013, and stated that no alcohol or drugs were found in his system. Lanza’s autopsy showed no tumors or gross deformities in his brain.

Details of the investigation were reported by law enforcement officials at a meeting of the International Association of Police Chiefs and Colonels held during the week of March 11, 2013. An article published in the New York Daily News on March 17, 2013, provided purported details of this report by an anonymous law enforcement veteran who had attended the meeting. The source stated that the investigation had found that Lanza had created a 7-by-4-foot sized spreadsheet listing around 500 mass murderers and the weapons they used, which was considered to have taken years of work and to have been used by Lanza as a “score sheet”. On March 18, 2013, Lt. Paul Vance of the Connecticut State Police responded that the information from this meeting was “law enforcement sensitive information” and considered the release to be a leak.

The final report of the State Attorney summarizing the investigation into the shooting was published on November 25, 2013. It concluded that Adam Lanza had acted alone, and that the case was closed. The report noted that “[Lanza] had a familiarity with and access to firearms and ammunition and an obsession with mass murders, in particular the April 1999 shootings at Columbine High School in Colorado.” The report did not identify a specific motive for the shooting, stating, “The evidence clearly shows that 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.”

On December 27, 2013, police released thousands of pages of documents pertaining to the investigation. In accordance with law, the names of victims and witnesses were redacted or withheld. The summary report included information about items found on Lanza’s computer equipment, including writings and material about previous mass shootings. A former teacher of Lanza’s noted that he exhibited antisocial behavior, rarely interacted with other students, and was obsessed with writing “about battles, destruction and war.”

In a 2013 interview, Peter Lanza said he suspected his son might have also suffered from undiagnosed schizophrenia in addition to his other conditions. Lanza said that family members might have missed signs of the onset of schizophrenia and psychotic behavior during his son’s adolescence because they mistakenly attributed his odd behavior and increasing isolation to Asperger syndrome. Because of concerns that published accounts of Lanza’s autism could result in a backlash against others with the condition, autism advocates campaigned to clarify that autism is a brain-related developmental disorder rather than a mental illness. The violence Lanza demonstrated in the shooting is generally not seen in the autistic population and none of the psychiatrists he saw detected troubling signs of violence in his disposition.

President Obama honored the six slain adults posthumously with the 2012 Presidential Citizens Medal on February 15, 2013. President Obama said “And then when Dawn Hochsprung, and Mary Sherlach, Vicki Soto, Lauren Rousseau, Rachel D’Avino, Anne Marie Murphy — when they showed up for work at Sandy Hook Elementary on December 14th of last year, they expected a day like any other — doing what was right for their kids; spent a chilly morning readying classrooms and welcoming young students — they had no idea that evil was about to strike. And when it did, they could have taken shelter by themselves. They could have focused on their own safety, on their own wellbeing. But they didn’t. They gave their lives to protect the precious children in their care. They gave all they had for the most innocent and helpless among us. And that’s what we honor today — the courageous heart, the selfless spirit, the inspiring actions of extraordinary Americans, extraordinary citizens.”

Within hours of the shooting, a We the People petition was started asking the White House to “immediately address the issue of gun control through the introduction of legislation in Congress,” and the gun control advocacy group the Brady Campaign to Prevent Gun Violence reported that an avalanche of donations in the hours after the shooting caused its website to crash. Five days later, President Obama announced that he would make gun control a “central issue” of his second term, and he created a gun violence task force, to be headed by Vice President Joe Biden. On January 16, 2013, Obama signed 23 executive orders and proposed 12 congressional actions regarding gun control. His proposals included universal background checks on firearms purchases, an assault weapons ban, and a high-capacity magazine ban limiting capacity to 10 cartridges.

On January 16, 2013, New York became the first U.S. state to act after the shooting when it enacted the Secure Ammunition and Firearms Enforcement (SAFE) Act. On April 3, 2013, Connecticut General Assembly passed a 139-page major gun-control bill with broad bipartisan support. Governor Dannel P. Malloy signed the bill on the same day. The bill requires universal background checks (background checks for all firearm purchases), a high-capacity magazine ban banning the sale or purchase of ammunition magazines capable of holding more than ten rounds of ammunition like those used in the Sandy Hook Elementary School shooting, created the first registry in the United States for dangerous-weapon offenders, and added over 100 types of gun to the state’s assault weapons ban. Pro-gun groups had rallied outside the Capitol to protest prior to the signing and challenged it in court. Federal judge Alfred Covello ruled in January 2014, to uphold the law.

On April 4, 2013, Maryland also enacted new restrictions to their existing gun laws. Ten other states passed laws that relaxed gun restrictions.

Legislation introduced in the first session of 113th Congress included the Assault Weapons Ban of 2013 and the Manchin-Toomey Amendment to expand background checks on gun purchases. Both were defeated in the Senate on April 17, 2013.

Police found numerous video games in the basement of Adam Lanza’s home, which was used as a gaming area. The final report into the shooting by the State Attorney, published in November 2013, noted that “[Lanza] played video games often, both solo at home and online. They could be described as both violent and non-violent. One person described the shooter as spending the majority of his time playing non-violent video games all day, with his favorite at one point being Super Mario Bros.”

The school was closed indefinitely following the shooting, partially because it remained a crime scene. Sandy Hook students returned to classes on January 3, 2013, at Chalk Hill Middle School in nearby Monroe at the town’s invitation. Chalk Hill at the time was an unused facility, refurbished after the shooting, with desks and equipment brought in from Sandy Hook Elementary. The Chalk Hill school was temporarily renamed “Sandy Hook”. The University of Connecticut created a scholarship for the surviving children of the shootings.

On May 10, a task force of twenty-eight appointed members voted to demolish the existing Sandy Hook Elementary school and have a new school built in its place. The $57 million proposed project was sent to the Newtown Board of Education for approval, to be followed by a public ballot. In October 2013, Newtown residents voted 4,504–558 in favor of the proposed demolition and reconstruction, to be funded by $50 million in state money. Demolition began on October 25 and was completed in December 2013 at a cost of nearly US$1.4 million.

According to a report by the Office of the Child Advocate in Connecticut in November 2014, Lanza may have suffered from anorexia nervosa. The authors wrote that “Anorexia can produce cognitive impairment and it is likely that anorexia combined with an autism spectrum disorder and OCD compounded Lanza’s risk for suicide”. They also noted that at the time of his death, Lanza “was anorexic (he was six feet tall (183 cm) and weighed 112 pounds (51 kg)), to the point of malnutrition and resultant brain damage.”

In March 2014, the Newtown city officials announced the design for the new Sandy Hook Elementary School. The only remnant of the original school would be its flagpole.

On December 15, 2014, nine families of the 26 victims of the shooting filed a class-action lawsuit in Connecticut against Bushmaster, Remington Arms, Camfour, a distributor of firearms, and the now-closed East Windsor store, Riverview Sales, where the gunman’s rifle was purchased, seeking “unspecified” damages, claiming an exemption from the 2005 Federal Protection of Lawful Commerce in Arms Act (PLCAA) that would normally disallow such a suit as lacking standing. The plaintiffs allege that the XM15-E2S is suitable only for military and policing applications and that Bushmaster inappropriately marketed it to civilians. In January 2015 Bushmaster’s attorneys petitioned to have the suit moved to federal court because, although the shooting took place in Connecticut, it is located in North Carolina. In February 2015 the victims’ families’ attorneys made a motion to move the suit back to state court. On April 14, 2016, a Connecticut court denied the defendants’ motion to summarily dismiss the case. Lawyers for the defense filed a second motion for dismissal a month later. On October 14, 2016, the defendants’ motion to dismiss the lawsuit was granted. The judge ruled the complaint was not valid per federal and Connecticut laws.

On October 21, building site preparation work began on the new Sandy Hook Elementary School; project updates and progress were posted on a dedicated website. Citing security and privacy reasons and out of respect for the families of victims, no ground breaking ceremony was held. Construction was scheduled to begin in March 2015 with the school expected to open by December 2016.

In December, it was announced that the town of Newtown will acquire the property and home of Nancy Lanza at no cost. The property at 36 Yogananda St. was part of the Lanza estate, to which surviving son Ryan Lanza is the sole heir. Lanza’s attorney, Kenneth Gruder, arranged for the transfer through a series of transactions so that probate records would not show the city acquiring the property from the Lanza family. Gruder said the notoriety of the home had made it essentially unsalable. The home was demolished in March 2015.

On January 21, 2015, Newtown Legislative Council voted unanimously to demolish the house where Nancy and Adam Lanza lived, and to keep the land as open space. The demolition was completed on March 24, 2015. Also in January, the families of two of the first-graders who died in the shooting filed a lawsuit against the city of Newtown and the Newtown Board of Education alleging inadequate security at the school.

In February 2015, the family of one of the victims, Victoria Soto, applied for trademark protection for her name. The reason for this was to help prevent others from misusing Soto’s name on social media and for the benefit of the memorial fund set up in her name. The victim’s sister, Jillian, stated that fake social media accounts existed using her sister’s name to promote conspiracy theories about the shooting.

In July 2016, the new Sandy Hook Elementary School was unveiled and parents were shown around the building. The new school is on the site of the building where the shooting took place, which was demolished and rebuilt with a state grant of around $50 million.

On April 17, 2018, radio host and conspiracy theorist Alex Jones was sued for defamation by three parents whose children were killed in the shooting. Prior to this, Jones said that the Sandy Hook shooting was “completely fake” and a “giant hoax” perpetrated by opponents of the Second Amendment. Jones claims he later believed the shooting was real. On May 23, 2018, six more families sued Jones for his claims. Jones was found to be in contempt of court by a Texas judge even before the trial started, failing to produce witnesses and materials relevant to the procedures. Consequently, Jones and InfoWars were fined a total of $126,000 in October and December 2019.

On October 16, 2019, a jury awarded Leonard Pozner $450,000 for defamation by James Fetzer, who had co-written the book Nobody Died at Sandy Hook. The book claimed that Pozner had fabricated the death certificate of his son Noah, a six-year-old victim of the shooting. Fetzer said he would appeal the decision.

The families appealed to the Connecticut Supreme Court. In March 2019 the court decided in a 4–3 vote to reverse parts of the trial court’s rulings and remand the case back to Bridgeport Superior Court for additional hearings. It ruled that the families’ appeal to the Connecticut Unfair Trade Practices Act, demonstrating that the gun manufacturers had used advertising that presented the weapons in an “unfair, unethical, or dangerous manner”, with Remington seeking to “expand the market for [its] assault weapons through advertising campaigns that encouraged consumers … to launch offensive assaults against their perceived enemies”, was not prohibited by PLCAA, and thus that the plaintiffs had sufficient standing to argue their case at trial court. It also ruled that the plaintiffs can subpoena internal documents on how gun companies have marketed the AR-15. Remington asked the Supreme Court of the United States to review the state court ruling, but in November 2019 the Supreme Court denied to hear the appeal, allowing the families’ suit to proceed.