Even before Adam Lanza was a teenager, it was clear something was very wrong. “The social awkwardness, the uncomfortable anxiety, unable to sleep, stress, unable to concentrate, having a hard time learning, the awkward walk, reduced eye contact,” his father said. “You could see the changes occurring.”

When he was 13, he was diagnosed with Asperger’s syndrome which, according to the American Psychiatric Association, falls under the broad umbrella of autism spectrum disorder. His parents, Peter and Nancy, were relieved.

They finally knew what was wrong with their shy, diffident son who would one day shoot and kill 20 children and six adults at Sandy Hook Elementary School in Connecticut. “It was communicated as ‘Adam, this is good news,'” Peter Lanza told the New Yorker. “This is why you feel this way, and now we can do something about it. ”

After Sandy Hook, Adam Lanza’s autism sparked a fiery debate that still burns today. Drawing any direct causality between his condition and the killings remains controversial. “To imply or suggest that some linkage exists is wrong and is harmful to more than 1.5 million law-abiding, nonviolent and wonderful individuals who live with autism every day,” the Autism Society said in a statement.

new study in the journal Aggression and Violent Behavior will likely fuel the debate even more. It found some mass murderers and serial killers have something in common: autism and head injury. Like Lanza, other killers had an inability to connect socially, spoke in flat monotones, and had emotionless features.

Researchers call it the first review of published accounts of what causes people to commit mass murder. They said: “Our findings tentatively indicate that these extreme forms of violence may be a result of a highly complex interaction of biological, psychological and sociological factors and that, potentially, a significant proportion of mass or serial killers may have had neurodevelopmental disorders such as autism spectrum disorder or head injury.”

The researchers stressed the study is “clearly limited” by the “anecdotal and speculative” nature of some of the published accounts. Lead researcher Clare Allely, of the University of Glasgow, emphasized the study did not suggest those with autism or Asperger’s are more likely to commit murder. “We’re not saying people with autism will be serial killers,” Allely said, adding “it’s way too early to make any statement like that.”

The director of the United Kingdom’s National Autistic Society’s Centre for Autism reacted to the study with caution. “We would urge people not to jump to conclusions about people with autism and to make judgements about a whole section of society,” Carol Povey told the Independent. “This and previous research shows that the vast majority of individuals with autism are law abiding and respect the rules of society. Indeed, in many cases, individuals with autism are unusually concerned to keep the letter of the law, due to the nature of the disability.

Calling such studies “vital,” she added: “This research reaffirms the importance of ensuring that people with autism get the support they need as early as possible.”

Nonetheless, “the findings were surprising,” researcher Allely, who holds a PhD in psychology, said in a phone interview. “There was a higher percentage of autism [among serial killers] than in the general population.” One in 68 American children today have autism, according to the U.S. Centers for Disease Control and Prevention.

The research examined a population of 239 killers who had murdered at least three people since 1985, when there was a dramatic increase in the recognition of autism. Of that number, 133 of the mass killers did not appear to suffer from any neurological disorder, but a “significant proportion” did: 28 percent had “definite, highly probable or possible” autism, while 21 percent had sustained a “definite or suspected head injury.” And of those 106 killers, more than half experienced some “psychological stressor” such as physical or sexual abuse.

One problem with the study: Different reporting methods for mass murder are found throughout the world. And “serial killing, in particular, may go unrecognized,” says the study, which combed numerous published accounts and public records.

But even with those limitations, reading the study is like a paging through a flip-book of recent horrors. There’s Seung Hui Cho, who killed 32 people at Virginia Tech. There’s Jared Loughner, who pleaded guilty to killing 19 people in Tuscon. And there’s one of the most infamous serial killer of all: Jeffrey Dahmer.

Serial killer Jeffrey Dahmer is seen in a 1982 Milwaukee county sheriff's department mugshot. (Handout via AP) Serial killer Jeffrey Dahmer in a 1982 Milwaukee county sheriff’s department mugshot. (Handout via AP)

Dahmer, who murdered 17 men and boys between 1978 and 1991, was never clinically diagnosed with autism, but evidence exists “to suggest he displayed numerous indications of Asperger syndrome,” the study said.

The Milwaukee killer was a loner as a child, unable to bond with anyone. He was “regarded as odd and bizarre,” the study said. “He had difficulty with nonverbal communication, such as a dearth of facial expression, and his unusual gaze.” His gait was “mechanical,” as though his “knees were locked” while walking.

Despite the patterns that emerged in the study, researchers cautioned against sweeping conclusions. Neurodevelopmental disorders, they said, do not portend mass murder.

“It is crucial to note that we are not trying to suggest that individuals with ASD or previous head trauma are more likely to be serial killers or commit serious crime,” Allely said. “Rather we are suggesting that there may be a subgroup of individuals within [mass killers] who may be more likely to commit serious crimes when exposed to certain psychosocial stressors.”