Kansas City Chief linebacker Jovan Belcher shot himself in front of his coach and general manager early Saturday, shortly after killing his girlfriend. Friends and teammates have claimed Belcher suffered from football-related head injuries, admitting he was also battling with depression and an addiction to pain killers.

Two days after the tragedy made headlines, researchers at Boston University’s Center for the Study of Traumatic Encephalopathy released a study highlighting 28 new cases of chronic brain damage in deceased football players—15 of whom played in the NFL.

The team examined 85 people with a history of mild to severe head injuries, including former high school, college and pro football players, boxers, NHL players and military veterans. Of those researched, 80 percent showed evidence of chronic traumatic encephalopathy (CTE) in their brain tissue. Depression, memory loss, dementia and cognitive problems are all symptoms of CTE, according to The Atlantic, which writes “at least six former NFL stars have committed suicide in the last two years and almost all had displayed some of the symptoms of CTE before their deaths.”

As part of the study, Boston University researchers were able to confirm the diagnosis in two of the deceased players, including former Chicago Bears player Dave Duerson who died last year by shooting himself in the chest, so his brain could be saved and donated to Boston University’s Center.

Earlier this year, Boston University tried to obtain the brain of former New England Patriot Junior Seau, after he also killed himself in the chest. The tragedy was reminiscent of Duerson’s, who wanted to see if the depression he suffered late in life could be linked to brain damage he suffered on the football field.

The study more than doubled the number of documented cases of CTE, according to the New York Times. The Center’s Co-Director Robert Cantu told PBS, “The sheer volume of the case, I think, is going to just overwhelm anybody that wants to be in denial about the existence of this problem.”

Several have criticized Boston University’s research, claiming it hasn’t yet identified a strong enough relationship between head trauma in football and CTE. As Ann McKee, the neuropathologist who led the study, told PBS, however: “I agree we don’t know how big a problem this is, we don’t know what all the risks are. There’s a lot we don’t know, but I think we know enough to know that this is a problem.”

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