On December 14, 2012, at Sandy Hook Elementary School in Newtown, Connecticut, Adam Lanza kills 20 first graders and six school employees before turning a gun on himself. Earlier that day, he killed his mother at the home they shared.

The Sandy Hook shooting was, at the time, the second-deadliest mass shooting in the United States after the 2007 shooting at Virginia Tech, in which a gunman killed 32 students and teachers before committing suicide.

Shortly after 9:30 a.m., 20-year-old Adam Lanza shot through a plate-glass window next to Sandy Hook’s locked front entrance in order to gain access to the school. Hearing the noise, the school principal and school psychologist went to investigate and were shot and killed by Lanza, who was armed with a semiautomatic rifle, two semiautomatic pistols and multiple rounds of ammunition. Lanza also shot and wounded two other Sandy Hook staff members.

He then entered two first-grade classrooms, where he gunned down two teachers and 15 students in one room and two teachers and five students in the other room. The children Lanza murdered, 12 girls and 8 boys, were 6 and 7 years old. Twelve first-graders from the two classrooms survived.

When Lanza heard the police closing in on him, he killed himself in a classroom at approximately 9:40 a.m.

Police soon learned that sometime earlier that morning, before arriving at Sandy Hook, Lanza had shot and killed his 52-year-old mother at their home. She owned the weapons her son used in his deadly rampage.

Investigators determined that Lanza, who had attended Sandy Hook as a boy, acted alone in planning and carrying out the attack, but they were unable to find a motive for his actions or discover why he had targeted Sandy Hook.

In November 2013, the Connecticut State’s Attorney released a report noting that Lanza had “significant mental health issues that affected his ability to live a normal life and to interact with others.” However, mental-health professionals who had worked with him “did not see anything that would have predicted his future behavior,” according to the report.

In the aftermath of the Sandy Hook shooting, President Barack Obama called for new gun-safety measures; however, his primary legislative goal, expanded background checks for gun buyers, was blocked by the U.S. Senate.

The community of Newtown, which has some 27,000 residents and is located about 45 miles southwest of Connecticut’s capital, Hartford, eventually decided to tear down Sandy Hook Elementary School. It was razed in the fall of 2013; a new school was built on the same site.