The Louisville bank shooter Connor Sturgeon had the nickname 'Mr Concussion' at school as he had been injured so many times playing sport.
A friend revealed Sturgeon had taken most of a year off playing American football when he was aged 13 to 14 because of suffering from multiple concussions.
He then went on to have a "couple more" concussions in high school, the friend said.
On Monday Sturgeon went into his work at the Old National Bank in Louisville, Kentucky, armed with an AR-15 rifle and fatally shot five colleagues.
Police rushed to the scene and the gunman was killed in a shootout with officers. One officer was shot in the head as he ran towards the shooter and he remains in critical condition.
Six people were killed, including the shooter, with nine being taken to hospital for treatment. The five deceased victims were identified as Joshua Barrick, 40, Thomas Elliot, 63, Juliana Farmer, 45, James Tutt, 64, and Deana Eckert, 57.
One of the dead includes a close friend of Kentucky's governor.
Now a former friend of the gunman and a teammate at Floyd Central High School said that the gunman suffered many head injuries.
He said Sturgeon always wore a helmet during basketball games because he had suffered so many concussions.
He told the Daily Beast : “The big thing I keep going back to is that in the first year of high school, we played football together in eighth grade, he was out most of the year because he had multiple concussions. Then he had a couple more in high school.
"I’m not saying it’s the cause but I always think back to that… There were times I’d wonder, will this catch up with him? But never in this way. He’s the last person I'd expect would do this.”
An Instagram account linked to Sturgeon had the username 'Mr. Concussion.'
Concussion is a type of traumatic brain injury (TBI), which has been linked to violent crime.
A 2018 Lancet paper found: "TBI appears to be associated with earlier age of incarceration, increased risk of violence, and more convictions. Neurological abnormalities are common in offenders."
Highlighting the connection between the latest mass shooter and repeated brain injuries, Dr Chris Nowinski CEO of Concussion Legacy Foundation wrote: "While nothing completely explains such a tragedy, it's time we recognize a pattern. We must do more to prevent brain injuries in kids."
According to the CDC: "A concussion is a type of traumatic brain injury—or TBI—caused by a bump, blow, or jolt to the head or by a hit to the body that causes the head and brain to move rapidly back and forth."
He was a track runner for his school in a Louisville suburb and was a semifinalist for a National Merit Scholarship in 2015, say local media reports.
Sturgeon, at 6ft 5in, was also a basketball player who went on to have a podcast with friends.
A former school classmate who claims to have known the gunman and his family well, said that there were no “red flags”.
Sturgeon joined the bank full-time in 2021 after three consecutive summer internships, and it is reported that he was recently fired from his job.
