Portrait of a killer: What drove Lanza to embark on his murderous spree?

Attempts to make sense of events will hinge on an understanding of the killer's psychology. So what do those who knew the 20-year-old think drove him to mass murder?

On the desks of police forces, politicians, lawyers and social analysts throughout the United States, one book, or its corresponding website, is likely to dominate attention over the coming days and weeks. The manual of the American Psychiatric Association, especially the section on personality disorders, will be scoured for insight into what went on inside the head of Adam Lanza, the 20-year-old from Connecticut who on Friday took the lives of 27 people, including 20 primary schoolchildren.

Lanza is dead: there is no way to test him for symptoms of paranoid or schizotypal illness; no way to gauge whether he suffered anti-social, narcissistic or avoidant disorders. Instead, what is left are the memories and experiences of those who knew the mass killer and the psychological jigsaw that will now be pieced together in the hope of coming up with a picture more complete than the descriptions that have emerged so far. These include: good smart kid; camera-shy nerd; a genius, and a remote goth. If there are answers to the big question – why did he do it? – they will come from looking beyond the comment from one family member who simply said "Adam was obviously not well", or a family friend, Barbara Frey, who offered: "He was a little bit different".

Lanza lived with his mother Nancy in a prosperous suburb on the edge of Newtown, Connecticut. The house is five miles from the scene of the massacre at Sandy Hook Elementary School. Authorities have not yet confirmed whether Lanza attended the junior school, but he was a student at the town's high school. Some classmates have described him as happy, and very smart. Joshua Milas, who hung out with him at the school's technology club, said he was probably a genius.

There are indications that high-functioning autism, or a degree of Asperger's Syndrome, could have been part of the behaviour that some interpreted as the eccentricity that went with his smartness. But nothing has been confirmed.

Other reports are contradictory, with classmates saying Lanza was a total loner and had no friends. Olivia DeVivo, once in Lanza's high school class and now at the University of Connecticut, told The New York Times: "I never saw him with anybody. Not one person." Other portraits paint a picture of someone painfully keen to avoid any attention, and nervous and fidgety when forced to communicate.

These reports do not match unconfirmed descriptions of a current or former girlfriend the police are anxious to speak to, and a so-far unnamed friend whom the authorities are trying to track down. But the real starting point in unravelling Adam Lanza's mind will be the shooting of his mother at home. He shot his mother in the face. He then drove her car to the school where reports said she had worked, possibly as a substitute teacher or voluntary classroom assistant.

A former FBI profiler, Clint Van Zandt, told the MSNBC channel that once Lanza had killed his mother he was capable of anything. But why did he then target the school and its young pupils?

According to other reports, Lanza was deeply upset at the divorce of his parents in 2009. Peter and Nancy Lanza were married for 28 years but the relationship had broken down by 2006, when Lanza was just into his teen years. Adam and his elder brother Ryan were said to be left depressed over their parents breaking up, though their mother was described as "working hard to protect her sons". Their aunt, Marsha, said her nephews were raised by nurturing parents who wouldn't have hesitated to call in psychological help if they thought it was needed. This contradicts the comment of a former babysitter at the Lanza home who described the young Adam as a "rambunctious" teenager, capable of severe mood swings and temper tantrums, who required medication to control his behaviour.

Neighbours in Stamford, Connecticut, where Peter Lanza now lives, said he had remarried recently. What effect this had on his son Adam will be one of the myriad factors considered by investigators.

An agreement of joint custody was reached over Adam, who was supposed to live primarily with his mother but to be permitted "liberal visitations and vacations" with his father. It is unclear if the split worked out as initially planned.

When the couple finally filed for divorce, a judge overseeing the formal dissolution of the marriage, ordered them to enrol in a "parenting education programme". This may have been routine for a departing father and a mother left to care for two young sons. But given the events on Friday, what the judge meant will be dissected in detail.

Mr Van Zandt believes the view from within the family may not reveal what Adam Lanza was seeing. "What did his [Lanza's] mother do? She was a kindergarten teacher. She had children that loved her and she loved them … When he shot his mother and turned on those children, those children were part of his mother. He killed what his mother loved."

This is the analysis that says Lanza chose ruthlessly to target what his mother cared about . He didn't flip in one day, but like others who left death behind at Columbine and Virginia Tech, planned his revenge carefully. Another former FBI profiler, Joe Navarro, based in Florida, suggests the authorities in Connecticut should be looking at the psychology of murder-by-proxy: that Lanza murdered 20 schoolchildren simply because of their emotional association with his mother.

The early confusion over the identity of the killer was because Adam had been carrying ID belonging to his elder brother. Ryan Lanza was contacted by police and offered what insight he could into what made his brother tick. Aged 24, living in Hoboken, New Jersey, having followed his father into a well-paid career as a tax accountant with Ernst & Young, Ryan told investigating officers: "My brother has always been a nerd."

The brothers hadn't met or been in contact for two years, suggesting their relationship was not close. But Ryan's own evaluation went further: he told the police that his brother was suffering from a "personality disorder".

The boys' father, formerly a tax specialist, is a vice-president at the investment company GE Capital, teaching occasionally at Northeastern University. He drove from his large family home in Stamford to talk to the police when he learnt about what had happened. Mr Lanza may have little insight into the last few years of his son's life, and police may instead be forced to rely on the accounts of those who claim to have known him over the past few years. At a vigil in Newtown on Friday night, police are said to have quietly approached those attending the gathering, asking for any small detail anyone could remember.

Catherine Orso said her son knew Lanza. "He just said he was very thin, very remote and was one of the goths." In the coming search for sense and reason in the aftermath of Friday's terrifying chaos, many will be asking themselves why they didn't look behind the goth mask or Lanza's loner persona. Ms DeVivo said: "I think he went so unnoticed that people didn't realise that maybe there's actually something else going on here – that maybe he needs to be talking or getting some kind of mental help. In high school no one really takes the time to look and think."

How the world reacted

"My thoughts are with the injured and those who have lost loved ones. It is heartbreaking to think of those who have had their children robbed from them at such a young age, when they had so much life ahead of them."

David Cameron, British Prime Minister

"I have been deeply shocked and saddened to learn of the dreadful loss of life in Newtown, Connecticut … Prince Philip joins me in extending our heartfelt sympathy to you and the American people at this difficult time. The thoughts and prayers of everyone in the United Kingdom and throughout the Commonwealth are with the families and friends of those killed and with all those who have been affected by today's events."

Queen Elizabeth II

"The Holy Father has asked me to convey his heartfelt grief and the assurance of this closeness in prayer to the victims and their families, and to all affected … In the aftermath of this senseless tragedy he asks God, our Father, to console all those who mourn and to sustain the entire community with the spiritual strength which triumphs over violence by the power of forgiveness, hope and reconciling love."

Cardinal Tarcisio Bertone, Pope Benedict XVI's spokesperson

"Once again we stand aghast at a deed that cannot be comprehended. The thought of the murdered pupils and teachers makes my heart heavy."

Angela Merkel, German Chancellor

"I was horrified by the news of the shooting today at Sandy Hook primary school in Newtown, Connecticut. Please accept my condolences on behalf of the French people and myself."

François Hollande, French President

"As parents and grandparents, as brothers and sisters, as friends of the American people, we mourn the loss of children, aged only five to 10 years, whose futures lay before them. We mourn the loss of brave teachers who sought only to lead their students into that future but were brutally murdered in a place of refuge and learning."

Julia Gillard, Australian Prime Minister

"Young lives full of hope have been destroyed. On behalf of the European Commission and on my own behalf, I want to express my sincere condolences to the families of the victims of this terrible tragedy."

Jose Manuel Barroso, President of the European Commission

"Vladimir Putin [would like to convey his] support and sympathy to the families and friends of the victims and [to express] his empathy with the American people."

Kremlin statement, On behalf of the Russian President

"The sympathy of the Japanese people is with the American people."

Yoshihiko Noda, Japanese Prime Minister

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