Adam Peter Lanza was born on April 22, 1992, in Kingston, New Hampshire. He lived with his mother, Nancy Lanza, and his older brother, Ryan. The family moved to Newtown, Connecticut, when Adam was a child. Growing up, Adam faced several developmental challenges. Before he turned three, he showed signs of communication issues, sensory difficulties, and socialization delays. His mother sought help through the New Hampshire Birth to Three program, which referred him to special education services.

In elementary school, Adam was diagnosed with a sensory-integration disorder. This disorder impacted his ability to process sensory information and was often associated with autism. His anxiety affected his schooling, and in eighth grade, he had to be placed on "homebound" status due to his difficulties.

At the age of 13, Adam was diagnosed with Asperger syndrome, a high-functioning form of autism. His parents later sought treatment for his obsessive-compulsive disorder (OCD) at Yale University's Child Study Center when he was 14. Adam exhibited behaviors typical of OCD, such as frequently washing his hands and becoming overly fixated on cleanliness. He struggled with the medication prescribed for his symptoms, leading to significant adverse effects that caused him to stop taking it after only a few days.

Adam attended Sandy Hook Elementary School and later Newtown Middle School, where he started in 2004. His mother reported that he experienced anxiety triggered by changing classrooms and loud noises. As a result, he transferred to a smaller parochial school, St. Rose of Lima, briefly before returning to Newtown High School. During high school, although he was recognized for his intelligence and was placed on the honor roll, he had difficulty socializing and is not known to have had close friends.

Struggles during adolescence intensified, and Adam attended school less frequently as he grew older. His mental health issues, including anxiety and depression, continued to worsen. He was sometimes home-schooled by his parents during these years. He also took some classes at Western Connecticut State University in 2008 and 2009.

In his final years, Adam became increasingly isolated. He would often spend time in his bedroom, engaging with the internet and video games. There were signs that his mental health was deteriorating. Reports later stated that he might have faced other severe mental health issues, which went undiagnosed, compounded by his isolation and obsessive thoughts about violence. Despite this, little treatment or support was provided to him during his life.

By December 2012, Adam was 20 years old, living with his mother in Newtown, while exhibiting behaviors that suggested he was deeply troubled. He had no known criminal record.

On December 14, 2012, Adam Lanza shot and killed his mother, Nancy Lanza, in their Newtown home before proceeding to Sandy Hook Elementary School. He used a .22-caliber rifle for this act. Nancy was found in bed with multiple gunshot wounds to her head.

Shortly after, Lanza drove to Sandy Hook Elementary School with his mother's Bushmaster XM15-E2S rifle. He arrived around 9:35 a.m. and shot his way through a glass panel next to the locked entrance doors. Dressed in black and wearing earplugs, he began shooting inside the school.

Principal Dawn Hochsprung and school psychologist Mary Sherlach, who were in a meeting, reacted to the sounds of gunfire. They went into the hall and encountered Lanza. Both were shot and killed. Natalie Hammond, a lead teacher, was also wounded but managed to hide and call for help.

Lanza entered two first-grade classrooms. In Room 8, he shot and killed 20 children and several adults. A young girl survived by hiding in a corner of the bathroom and later was found by police. In Room 10, Lanza continued his attacks, killing more children and teachers.

The police arrived soon after the shooting began, but by the time they reached Lanza, he had taken his own life with a gunshot. The entire incident unfolded in less than five minutes.

The aftermath of the shooting saw a significant police response, and the community grappled with the shock and tragedy of losing many young lives in one of the deadliest school shootings in American history.