Both men were white and similar in age, but what other shared traits might've pushed them to commit mass murder on a terrifying and indiscriminate scale?
For two years, The Violence Project, in conjunction with the National Institute of Justice, has compiled data on every mass shooter dating back to 1966 to examine potential patterns.
The project, funded by the US Department of Justice, aims to identify potential warning signs — red flags that could help prevent tragedies in the future.
FOUR COMMON TRAITS
The Violence Project has profiled the people behind every mass shooting — defined as an attack in which four or more people are shot and killed in a public place.
Among the perpetrators, researchers found four commonalities in nearly every instance.
In a piece for The Los Angeles Times on Sunday, psychologist and criminologist Jillian Peterson, together with colleague James Densleyk, a sociologist and criminal justice expert, detailed their findings.
Firstly, the majority of mass shooters they studied had experienced early childhood trauma as well as exposure to violence at an early age, they found.
"The nature of their exposure included parental suicide, physical or sexual abuse, neglect, domestic violence and/or severe bullying," they wrote.
Trauma suffered in childhood was often a precursor to mental illness conditions ranging from depression and anxiety through to thought disorders, the pair explained.
"Second, practically every mass shooter we studied had reached an identifiable crisis point in the weeks or months leading up to the shooting," they wrote.
A specific grievance led to the attackers feeling angry and despondent, but the triggers differed depending on the location of the shooting.
For attacks at workplaces, the perpetrators — often employees — were motivated by a change in job status, while in other places, relationship rejection or the loss of a relationship often played a role.
"Third, most of the shooters had studied the actions of other shooters and sought validation for their motives," Ms Peterson and Mr Densleyk explained.
"Societal fear and fascination with mass shootings partly drives the motivation to commit them. Hence, as we have seen in the last week, mass shootings tend to come in clusters. They are socially contagious."
In the wake of the Sandy Hook massacre in 2012, in which Adam Lanza opened fire at a primary school in Newtown, Connecticut, killing 26 people — including 20 children — investigators made a chilling discovery.
The 20-year-old had compiled detailed information on more than 500 violent crimes, meticulously studying virtually every detail of them.
The fourth common trait among shooters discovered by The Violence Project was one at the centre of the endless debate, reignited in America after each shooting.
Attackers each had the means to carry out their plans.
"In 80 per cent of school shootings, perpetrators got their weapons from family members, according to our data. Workplace shooters tended to use handguns they legally owned. Other public shooters were more likely to acquire them illegally," Ms Peterson and Mr Densleyk wrote.
Data shows that in 2018, illegal firearms weren't the most common guns used in attacks — just 10 of the 24 shootings in 2018 were carried out with illegally possessed weapons.
SIGNIFICANT STRESSORS COMMON
Each year, the US Secret Service produces a report into mass attacks in public spaces, exploring the backgrounds of those responsible.
In almost all of the 27 mass shootings in 2018, the criminals had experienced a significant stressor in their lives.
"Regardless of whether these attacks were acts of workplace violence, domestic violence, school-based violence or inspired by an ideology, similar themes were observed in the behaviours and circumstances of the perpetrators," its latest report reveals.
"Nearly all had at least one significant stressor within the last five years, and over half had indications of financial instability in that time frame."
Beyond a significant criminal charge, some of the most common stressors identified were family or relationship breakdowns, work or education setbacks, and domestic violence.
"Half were motivated by a grievance related to a domestic situation, workplace or other personal issue," the report continued.
A criminal history was a common trait among mass shooters, the report found.
"Approximately half of the attackers had histories of criminal charges beyond minor traffic violations. Those charges included both nonviolent and violent offences.
"Looking specifically at the issue of domestic violence, eight attackers (30 per cent) were found to have had such histories, with only some of those instances resulting in criminal charges or arrests."
Among the perpetrators of school shootings, the young offenders had also experienced a number of stressors, Ms Peterson said.
"Forty-five per cent had witnessed or experienced childhood trauma, 77 per cent had mental health concerns, as evidenced in a prior diagnosis, previous counselling or hospitalisation or medication use, and 75 per cent had an interest in past shootings, as evidenced in their writing, social media posts or other activities," she wrote in a piece for The Conversation.
The study by the Secret Service found two-thirds of attackers in 2018 had experienced mental health issues prior to the incident.
The most common illnesses noted among perpetrators were depression and psychotic symptoms like delusions and paranoia.
However, less than half of those had received mental health treatment.
More broadly, Claire Allely, a psychology lecturer at the University of Salford, said research of a larger pool of shootings had made similar conclusions.
"Mass shooters tend to have common psychological and behavioural characteristics such as depression, resentment, social isolation, the tendency to externalise rather than internalise blame, fascination with graphically violent entertainment, and a significant interest in weaponry," Ms Allely wrote for The Conversation.
Relying on mental illness as a red flag is flawed, though.
Obviously, not all people experiencing mental health issues are prone to carry out mass shootings merely because mass shooters battle mental illness.
"These psychological and behavioural characteristics are fairly prevalent in the general population," Ms Allely said.
"For example, the National Alliance on Mental Illness in the US says that about 20 per cent of Americans suffer with mental health disorders in any given year — that's more than 60 million individuals."
In a similar vein, claims in the past that school shooters tended to be autistic or have Asperger's syndrome are highly flawed.
Ms Allely led research on this common and frequently repeated claim, examining 75 mass shooters for the presence of autism spectrum disorders.
"In this study, we found six cases — or 8 per cent of the total number of mass shooters in the sample — who either had a diagnosis of autism or whose family and friends suspected they had an autism spectrum disorder," she said.
"The findings don't suggest that people with autism are more likely to become mass shooters."
GENDER, AGE AND RACE
In 2018, the majority of shooters were male and white, and while ages ranged from 15 to 64, the average was 37.
Examining a larger pool of shootings, Ms Allely said in 95 per cent of cases, the perpetrator was male and two-thirds were caucasian.
"Research shows that half of mass shooters are older than 30, with just 12.2 per cent under the age of 20, and 38 per cent between the ages of 20 and 29," she said.
In her analysis, Ms Peterson said there were similar gender and race characteristics among the perpetrators of school shootings in the US.
"All of the (kindergarten to year 12) school shooters or would-be school shooters were male and between the ages of 12 and 17," Ms Peterson wrote in The Conversation.
"The majority were white and nearly all — 91 per cent — were students or former students at the targeted school."
VIOLENT VIDEO GAMES
In recent days, a number of politicians in the US have blamed ultra-violent video games for prompting mass shooters to carry out attacks.
In 2013, in the wake of the Sandy Hook massacre, the town of Southington in Connecticut introduced a game buyback scheme.
Parents who wanted to get rid of their kids' violent games received a $25 gift card in exchange for dumping them.
The idea was prompted by rhetoric from politicians as well as a fierce campaign by the National Rifle Association — the powerful gun lobby group — attacking the video game industry.
It was a similar tactic the NRA employed after the Columbine school shooting.
But do video games inspire mass murder among young men?
A 2010 study analysed the findings of 130 different studies on the impact of video game violence.
They suggested there was some evidence that long-term exposure to very violent games could lead to increased aggression in real life but noted the effects were usually minor in nature.
However, Andy Ruddock, a media researcher and lecturer at Melbourne's Monash University, said the findings were "socially significant".
"In other words, gaming violence isn't the major cause of real-world violence, but it probably is enough of a catalyst to warrant concern," he wrote in The Conversation.
NO SINGLE MOTIVE
The difficulty in preventing future mass shootings is that no single cause or motive appears to have been behind past public attacks, the Secret Service report found.
Instead, a combination of factors seem to combine in a kind of perfect storm. The cause of optimism from the report is that "we can identify warning signs".
"The violence in this study resulted from a range of motives, with some attackers having multiple motives. In half of the incidents, grievances appeared to be the main motivating factor."
In 22 per cent of those cases, attackers were retaliating for perceived wrongdoings in domestic situations, while 11 per cent were motivated by workplace issues.
"Beyond grievances, some motives were related to the attackers' mental health symptoms (19 per cent), while others were connected to ideological beliefs (7 per cent)."
And despite the publicity given to Islamic terrorism over the past several years, the ideologies identified in 2018 were anti-abortion and anti-Semitic in nature.
"While only two of the attacks were primarily motivated by an ideology, nearly one-third of the attackers appeared to have subscribed to a belief system that has previously been associated with violence," the Secret Service report said.
"Often the attackers' beliefs were multifaceted and touched on a range of issues, including white supremacy, anti-Semitism, conspiracy theories, sovereign citizens, animal rights and the 'incel' movement."
Incels are men who identify as "involuntarily celibate" and are part of an internet-based subculture who believe they re entitled to sexual or romantic relationships but aren't desired by women.
WARNING SIGNS PRESENT
The Violence Project analysis found that in many cases, perpetrators who reached a "crisis point" before an attack had in many cases left clues prior to the shootings.
Crises were often communicated through "a marked change in behaviour, an expression of suicidal thoughts or plans or specific threats of violence".
Likewise, the Secret Service report noted that more than three-quarters of the attackers in 2018 had made threatening or concerning communications, and a similar number had elicited concern from others.
"The violence described in this report is not the result of a single cause or motive," it said.
"The findings emphasise, however, that we can identify warning signs prior to an act of violence.
"While not every act of violence will be prevented, this report indicates that targeted violence may be preventable if appropriate systems are in place to identify concerning behaviours, gather information to assess the risk of violence and utilise community resources to mitigate the risk."
