Video: 'LIVE: State police in CT address school shooting'

(Friday, December 14, 2012, approximately 9:35-9:40 a.m. EST; during the Sandy Hook Elementary School shooting ) — A gunman with a semi-automatic rifle killed 20 first-graders and six adult staff members today at Sandy Hook Elementary School in Newtown, Connecticut, then took his own life as police arrived.

Witnesses and officials described a horrific scene as the gunman, with brutal efficiency, chose his victims in two classrooms while other students dove under desks and hid in closets.

Hundreds of terrified parents arrived as their sobbing children were led out of the Sandy Hook Elementary School in a wooded corner of Newtown.

By then, all of the victims had been shot and most were dead, and the gunman, identified as Adam Lanza, had committed suicide.

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The children killed were said to be 6 to 7 years old.

Earlier that day, before driving to the school, Lanza shot and killed his mother, Nancy Lanza, at their Newtown home.

A November 2013 report issued by the Connecticut State Attorney’s office concluded that Lanza acted alone and planned his actions, but provided no indication why he did so, or why he targeted the school.

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

Video: 'Mass Shooting at Sandy Hook Elementary School | Flashback | NBC News'

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.”

The incident is the deadliest mass shooting at an elementary school in U.S. history, and the fourth-deadliest mass shooting overall.

The shooting prompted renewed debate about gun control in the United States, including proposals to make the background-check system universal, and for new federal and state gun legislation banning the sale and manufacture of certain types of semi-automatic firearms and magazines which can hold more than ten rounds of ammunition.