At least 49 revellers at a Florida gay club were brutally gunned down in the worst shooting in US history, sending shockwaves around the world.
Omar Mateen stormed Pulse nightclub in Orlando at around 2am on Sunday morning. He was killed in a gunfight with police after a three-hour siege.
ITV News Central spoke to Prof Craig Jackson, head of psychology at Birmingham City University, who specialises in researching and analysing spree killings, to ask him what his research had exposed about the minds of mass murderers like Mateen.
- What makes a mass killer?
Prof Jackson said often, the type of person who sets out on a spree killing is an “underachiever”, who tends to negatively compare themselves to others around them.
On top of this, they usually show signs of narcissism and conceit, seeing themselves as more important in the world than they really are.
“Coupled with their lack of achievement, they become quite bitter, ostracised, and alienated from society,” he said.
A stressful ‘trigger’ event usually precedes an assault of this type, he added - relationship breakdown, struggling at work, trouble with police - often within 24 hours of the attack.
Prof Jackson said the evidence shows that mental health problems are not directly linked to mass killings, despite what many might believe.
Instead, he said, it is a disordered or destructive personality type which means they cannot accept blame, and places responsibility for their circumstances in life onto others - often minority groups or communities.
“When we get all of these ingredients coupled together and in the right order and in the wrong circumstances, we see these killers act out their vengeful fantasies,” he said.
And even though they are the ones committing murder, they often still see themselves as the victim, he added.
“We tend to find that those who end up undertaking killing sprees have a very fixed personality, very fixed views in life and very fixed ways of doing things. What that often means is they get overtaken by other people - those who are flexible, or can adapt to new values or customs in society do better in jobs and in life.
“That can lead to feelings of being victims, but also hatred and anger.”
- Why does America suffer so many mass shootings?
There has been an acceleration in the number of high-fatality spree killings since the 1980s, Prof Jackson said - partly because of an increased number of people in the west feeling disenfranchised, but also due to the ready availability of high-powered assault weaponry.
It is legal in most parts of the US for anyone to own powerful assault rifles - and a striking fact about mass shootings is that it is usually amateurs who are behind the biggest death tolls - the people who enter clubs or theatres and simply open fire.
Prof Jackson said he has found the shooters with experience - for example, military-trained gunmen - tend to choose more open spaces, where they can pick their victims from a distance.
But access to guns is only part of the story. Canada has a similar level of gun ownership as the US, and suffers far fewer mass shootings.
Anger and issues of vengefulness are key issues for people in the US, which, Prof Jackson said, was largely forged through the use of deadly weapons and force.
“That attitude and the anger that we see in many North American spree killers is reminiscent of that ‘Wild West’ attitude,” he said.
“We find that many spree killers are angry with the community, with society at large, and they feel that killing children, or innocents, or shoppers, or co-workers, is the best and most efficient way to get back at that community.
“They often leave behind manifestos… saying they’ve had to resort to a spree killing because there was no other way for people to listen to them.”
- Why was the LGBT community targeted?
Relations with members of the LGBT community have become increasingly fraught in recent months, with major controversy over a warning from President Barack Obama to schools that they must allow transgender students to use whichever bathroom they felt most comfortable with.
But to understand why Mateen directed his attack towards this community in particular, Prof Jackson said more needed to be known about his background.
“We’ve already heard from the killer’s father that he had a problem with gay people and that seeing gay men kissing made him very angry,” he said.
“There could be further back in his occupational or schooling history where he was overtaken by people who were gay or were bisexual, and may have borne a grudge.”
What’s more, he added, by targeting the LGBT community, Mateen was also sending a “very clear message” to society as a whole - that their acceptance of homosexuality was something he also objected against.
- What can be done to prevent these attacks in future?
We are only just beginning to understand the psychological make-up of the kind of person who commits these kinds of mass shootings, Prof Jackson said, meaning preventative measures are difficult to identify.
Most police forces issue guidance videos on how to behave in an assault to maximise chances of survival.
On a practical level, he said, it is incredibly unlikely that we will ever see the end of US citizens’ constitutional ‘right to bear arms’.
However, one small measure which might be helpful would be to cease trade in large-capacity magazine rounds, which allow shooters to unleash a hail of bullets for a sustained period of time, without having to pause to re-load.
“If these were not available, when an individual does embark on a spree, they would run out of ammunition much more quickly, and have to re-load more often,” he said.
“That would give those caught up in the situation more of a chance to escape or overpower them.”
He said he believes more research is also needed into what factors can prompt an otherwise “perfectly normal” person - someone who may not even have been in trouble with police before - to “suddenly flip”, grab a gun and start taking lives.