The sports world was shocked in 2013 when Aaron Hernandez, star tight end for the New England Patriots, was arrested and charged with first-degree murder. But dating back to his days as a heavily recruited college athlete, there had been signs of ominous changes in his behavior: paranoia, failed drug tests, and run-ins with the law. And after Hernandez's death, some wondered if those changes could be attributed to a neurological condition called CTE.

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