TRUE CRIME: Inside the life of Adam Lanza, the man responsible for the Sandy Hook massacre.

Adam Lanza was quiet and fidgety, with brown hair that never sat quite right.

His thin face only emphasised his perpetual expression of worry, his blues eyes always staring intensely at a world he did not seem to be able to make sense of.

Adam was not even three when his parents recognised something unusual. He suffered developmental delays, struggling with communication and any sort of socialising, and engaged in unusual repetitive behaviours.

For four and a half years, Adam attended Sandy Hook Elementary School, where he was diagnosed with a sensory-integration disorder. The condition does not hold official status in the medical community, but is widely understood to be one of the leading characteristics of autism. Years later, as an adolescent, he would be diagnosed with Asperger syndrome.

The constant in Adam’s life, even from childhood, was anxiety.

According to his mother Nancy, he was “wracked by anxiety”, unable to cope with moving classrooms or loud noises. As a child, it became so overwhelming, his mother had to take him to the emergency room.

As Adam grew into adolescence, his parents noticed he was washing his hands incessantly, and over the course of a day would change his socks upwards of 20 times and go through a box of tissues, given he could not touch a doorknob with his bare hands. At 14, he was officially diagnosed with obsessive-compulsive disorder, a mental illness typified by unreasonable thoughts and fears that manifest in unhealthy patterns of behaviour.

Eventually, Adam’s mental state deteriorated so severely that he was placed on “homebound” status – meaning that he was unable to attend school, even with programs, support and appropriate accommodations.

This just further reinforced his sense of isolation.

In documents released this week by the FBI, a number of details about Adam’s life were made public.

At about 15 years old, Adam was visited at his home by federal agents after he hacked into a confidential government computer.

LISTEN: The recent massacre in Las Vegas was the deadliest mass shooting in modern history. Post continues below. 

His mother tried to convince officials that he was, “just very intelligent and was challenging himself to see if he could hack [his way in].”

According to the report, the authorities said that if he was that smart, perhaps he could get a job with them one day.

One woman, who is not named in the documents, described him as “the weirdest person online…” who “devoted almost all of his Internet activity to research and discussing mass killings.” She said he was “extremely intelligent,” but simultaneously depressed and highly cynical of what he saw as a “broken society”. The same woman said he became increasingly preoccupied with school shooters, and viewed them with “respect and understanding”.

His mother, it is reported, was a gun enthusiast who “loved the feeling and power of a gun in her hand.” There were a number of weapons accessible within the house, and according to close friends of Nancy, she never harboured concerns or was scared of her adult son.

In the months before the deadliest school shooting in US history, Adam fell deeper into depression. He would sleep for more than 12 hours a night, rarely ate, complained that natural light stung his eyes and forbade his mother from entering his bedroom. He told an online friend that he thought he was “asexual”.

Another source told the FBI that prior to the shootings he told her on a ‘suicide website’ that he planned on ending his life and it would “be on the news”.

And Adam Lanza, on Friday the 14th of December, 2012, did take his own life. But not before taking 27 others.

At 20 years old, Adam first shot dead his 52-year-old mother some time before 9:30am in the home they shared. Police found her body lying in bed, still with her pyjamas on, with four gunshot wounds to her head.

He then drove to Sandy Hook Elementary School, where the school day had just commenced.

Wearing black, with yellow earplugs and sunglasses, Adam shot down the locked entrance doors of the school. Principal Dawn Hochsprung was among those who heard the shots, but did not understand what they were. Moments later, she was murdered.

Adam then forced his way into a Year One classroom, being taught by Lauren Rousseau, a substitute teacher. She was in the process of trying to hide them when he entered. Adam killed Rousseau, a teacher assisting her, and 15 of her students.

The massacre continued, with Adam kicking down classroom doors and terrorising screaming children. He shot at students hiding under their desks. One teacher, Victoria Leigh Soto, forced students into bathrooms and closets, and lied to Adam about their whereabouts. Ultimately, she put herself between him and the students, and was shot dead as a result.

At 9:40am, only five minutes after breaking down the doors of Sandy Hook Elementary School, Adam fired his final shot into the back of his own head.

The 20 children who were murdered were all between the ages of six and seven. Six female teachers also lost their lives.

That's the point, in the wake of Sandy Hook, the FBI should be highlighting.

If you think you may be experiencing depression or another mental health problem, please contact your general practitioner or in Australia, contact Lifeline 13 11 14 for support or beyondblue 1300 22 4636.

You can listen to the latest episode of Mamamia Out Loud, here.