The gunman who carried out the worst mass shooting in Maine history had suffered severe brain damage that could stem from exposure to training blasts in the military, a study found.

A post-mortem analysis of Robert Card's brain by Boston University's CTE Center, completed at the request of the Maine Chief Medical Examiner’s Office, revealed "significant evidence of traumatic brain injuries at the time of the shootings." Card's family made the findings public and declined to comment.

Among the injuries recorded by researchers were damage to the fibers that allow communication between areas of the brain, inflammation and a small blood vessel injury, according to the report signed by Dr. Ann McKee, director of the lab at Boston University, and released Wednesday. She said there was no evidence of chronic traumatic encephalopathy (CTE), a brain disease often found in athletes and military veterans who have suffered repetitive head trauma.

"While I cannot say with certainty that these pathological findings underlie Mr. Card’s behavioral changes in the last 10 months of life, based on our previous work, brain injury likely played a role in his symptoms," McKee said.

The lab report showed that white matter in the brain had "moderately severe" damage and was missing entirely in some areas, according to The New York Times, which first reported the story. The tissues meant to insulate biological circuits were in "disorganized clumps." The rest of Card's brain also showed scarring and inflammation, which suggests repeated trauma.

Dr. Lee Goldstein, professor of neurology at Boston University who analyzed the brain tissue, told the newspaper cablelike cells that facilitated communication deep in the brain had lost protective wrapping and in some instances were missing or "filled with cellular garbage bags."

Although she didn't examine Card's brain or view the scans, Kristen Dams-O'Connor, director of the Brain Injury Research Center at Mount Sinai, said the descriptions available provide "compelling" evidence the clinical consequences he experienced came from brain injury.

"Without hard evidence defining 'safe' levels of blast exposure − if there is such a thing − we can't prevent this outcome, and these families won't be the last to be left heartbroken," she said. "There is an urgent need to address the question of  'how much is too much' when it comes to blast exposure."

The brain tissue sample was sent to the lab last fall by Maine's chief medical examiner. At that time, a Pentagon spokesperson said the Army was working to better understand the relationship between “blast overpressure” and brain health effects and had instituted several measures to reduce soldiers’ exposure, including limiting the number of personnel near blasts. 

On Oct. 25, the 40-year-old Army reservist opened fire at a bar and bowling alley in Lewiston, killing 18 people and wounding 13. Days later, after an intense search that kept residents across the city locked in their homes, authorities found Card dead of a gunshot wound.

Card is believed to have been exposed to thousands of blasts during his time as an instructor at a hand grenade training facility, the report says. His family said in a statement that it released the findings in part to "raise awareness of traumatic brain injury among military service members" and "encourage more research and support for military service members with traumatic brain injuries."

Over months, Card's family and fellow servicemen reported his increasingly erratic behavior to authorities, saying he was hearing voices and possibly stashing guns and that he had punched a soldier and threatened to shoot up a National Guard facility.

Card was taken to a psychiatric hospital for two weeks of treatment in July 2023. But even after he was released, there were several other reports of violence and alarming statements involving him to Army officials and local law enforcement in the two months before the shooting.

In their statement accompanying the brain study, Card's family apologized to the victims and their families.

"We are hurting for you and with you, and it is hard to put into words how badly we wish we could undo what happened," the statement said. "We know it does not fully explain Robert’s actions, nor is it an excuse for the horrific suffering he caused."

Contributing: The Associated Press

Adrianna Rodriguez is a health reporter for the USA TODAY nation team. Contact Adrianna at [email protected] or @AdriannaUSAT on X.