A report issued by the Office of the Child Advocate in November 2014 said that Lanza had Asperger's syndrome and, as a teenager, suffered from depression, anxiety, and obsessive-compulsive disorder, but concluded that these factors "neither caused nor led to his murderous acts".

The report went on to say, "his severe and deteriorating internalized mental health problems [...] combined with an atypical preoccupation with violence [...] (and) access to deadly weapons [...] proved a recipe for mass murder.

[31][35] Shortly after 9:35 a.m., armed with his mother's Bushmaster XM15-E2S rifle and ten magazines with 30 rounds each,[2][3][4][36][37] Lanza shot his way through a glass panel next to the school's locked front entrance doors.

[67] Anne Marie Murphy, the special education teacher who worked with special-needs students in Soto's classroom, was also shot and killed; she was found covering six-year-old Dylan Hockley, who also died.

[70] According to the official report released by the state's attorney, nine children ran from Soto's classroom, and police found two hiding in a class bathroom.

Following a lockdown drill weeks earlier, Roig had failed to remove a piece of black construction paper covering the small window in her classroom door.

[116] At a press conference on December 15, Dr. H. Wayne Carver II, the Chief Medical Examiner of Connecticut, was asked about the wounds, and replied "All the ones that I know of at this point were caused by the long weapon.

[125] 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.

[127] The March 28 documents also provided details on items found at Lanza's home, including three samurai swords, a newspaper article about the Northern Illinois University shooting, and a National Rifle Association certificate.

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 found no evidence that Lanza had taken drugs or medication that would have affected his behavior, and observed, "'Why did the shooter murder twenty-seven people, including twenty children?'

Rather there was a cascade of events, many self-imposed, that included: loss of school; absence of work; disruption of the relationship with his one friend; virtually no personal contact with family; virtually total and increasing isolation; fear of losing his home and of a change in his relationship with Mrs. Lanza, his only caretaker and connection; worsening OCD; depression and anxiety; profound and possibly worsening anorexia; and an increasing obsession with mass murder occurring in the total absence of any engagement with the outside world.

They wrote: "It is fair to surmise that, had Lanza's mental illness been adequately treated in the last years of his life, one predisposing factor to the tragedy of Sandy Hook might have been mitigated".

The report from the Office of the Child Advocate stated: "In the course of Lanza's entire life, minimal mental health evaluation and treatment (in relation to his apparent need) was obtained.

Of the couple of providers that saw him, only one—the Yale Child Study Center—seemed to appreciate the gravity of (his) presentation, his need for extensive mental health and special education supports, and the critical need for medication to ease his obsessive-compulsive symptoms.

[174] His computer contained two videos of gunshot suicides, movies that showed school shootings, and two pictures of Lanza pointing guns at his own head.

[179] 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.

[191] 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,[192] 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.

It ruled[202] 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.

"[233] The Sandy Hook Promise Foundation, a 501(c)(3) nonprofit organization, was formed to help survivors and families of the victims while also continuing the service of assisting others impacted by other shootings across the United States.

[234] President Barack Obama gave a televised address on the day of the shootings: "We're going to have to come together and take meaningful action to prevent more tragedies like this, regardless of the politics.

Malloy said, "Evil visited this community today, and it is too early to speak of recovery, but each parent, each sibling, each member of the family has to understand that Connecticut, we are all in this together, we will do whatever we can to overcome this event, we will get through it.

[258][259] 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.

[271] Legislation introduced in the first session of 113th Congress included the Assault Weapons Ban of 2013[272][273] and the Manchin-Toomey Amendment to expand background checks on gun purchases.

One person described the shooter as spending the majority of his time playing non-violent video games, with his favorite at one point being Super Mario Bros."[10] The report described his liking for Dance Dance Revolution, which he played frequently for long stretches of time at a movie theater in Danbury which had a commercial version of the game, and also owned a console version of at home.

[66][10] However, the Report of the Child Advocate said "video game and internet addiction appear to be 'highly comorbid with several other psychiatric disorders' including anxiety, depression, and obsessive compulsive challenges".

"[287] On October 21, building site preparation work began on the new Sandy Hook Elementary School;[288] project updates and progress were posted on a dedicated website.

[292] 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 design of the new school incorporates numerous security features, including bulletproof glass exteriors, fencing around the perimeter of the building, and fortified safe rooms.