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Researchers studying the brains of 202 former football players discovered traces of chronic traumatic encephalopathy (CTE) in nearly all of them, with the highest instances occurring in players who had reached the professional level.

While the report, which was published Tuesday in the Journal of the American Medical Association, doesn’t confirm that the condition is common in all football players, the Boston brain bank researchers diagnosed CTE in 177 former players, or nearly 90 percent of the samples studied. Many donors or their families contributed to the studies because of the players’ repeated concussions and symptoms before death, The Associated Press reported.

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CTE is a brain disease linked with repeated head trauma and is a concern among many for professional athletes as well as combat veterans.

Of the 111 brains of former NFL players, researchers discovered evidence of CTE in 110. It was found in 48 of 53 college players, nine of the 14 semi-professional players, seven of eight Canadian Football league players and three of 14 high school players. Researchers found no trace of CTE in two brains from younger players.

“There are many questions that remain unanswered,” Dr. Ann McKee, a Boston University neuroscientist, told the Associated Press.

McKee said among those questions is the prevalence of CTE in the general population as well as football players at every level, and what is the threshold for risk. She also said researchers are uncertain if lifestyle factors, like drugs, steroids, alcohol and diet might contribute.

“How many years of football is too many? What is the genetic risk? Some players do not have evidence of this disease despite long playing years,” McKee said.

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The NFL issued a response to the report saying that the league “will continue to work with a wide range of experts to improve the health of current and former NFL athletes.” Many former NFL greats were included in the study, namely Bubba Smith, Ken Stabler, Junior Seau, Frank Wainright and Dave Duerson.

“A lot of families are really tragically affected by [CTE] – not even mentioning what these men are going through and they’re really not sure what is happening to them,” Wainright’s widow, Stacie, told The Associated Press. “It’s like a storm that you can’t quite get out of.”

Wainright was 48 when he died in October of a heart attack, which Stacie said was triggered by a brain bleed. He spent 10 years in the NFL before retiring, and spent the last eight of his life suffering from confusion, memory loss and behavior changes. Stacie said her husband was adamant about donating his brain to science over fears of CTE.

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More than 30 former NFL players have pledged to donate their brain to science after death, including Hall of Fame defensive tackle Warren Sapp, Pro Bowl performers Randy Cross, Keith Sims and Shawn Springs. Almost 1,500 former athletes and military veterans have pledged to the do the same since 2008.

“There’s no way any of us want to really admit that we can’t remember how to get home or a grocery list that the wife has given us or how to go pick up our kids to the school, or whatever it may be,” Sapp said in a letter posted to The Player’s Tribune. “You try ‘Alright, I’m gonna get a little more sleep. Maybe it’s something I did last night, maybe something I drank’ or whatever it is.”

“You try to find a reason that it’s not ‘It’s my brain.’ That I’m not deteriorating right before my own eyes,” Sapp wrote. “It’s the most frightening feeling, but it’s also a very weakening feeling because you feel like a child. I need help I need somebody to help me find something that I could’ve’ found with my eye’s closed, in the dead of night, half asleep.”

The Associated Press contributed to this report.