Aaron Hernandez, the former New England Patriots star who took his own life in April while serving a life sentence in prison, was reportedly suffering from a degenerative brain disease.
Chronic traumatic encephalopathy is believed to be caused by repetitive brain trauma, according to the Boston University CTE Center.
"This trauma includes both concussions that cause symptoms and subconcussive hits to the head that cause no symptoms," explains the center's website.
The number or type of hits to the head needed to trigger CTE is still unknown, the center says, noting that other factors, such as genetics, may play a role in the development of the disease, but "these other factors are not yet understood."
Since it was first discovered in 2002 in the post-mortem brain of Pittsburgh Steeler star Mike Webster, the disease has been found in the brains of an increasing number of dead football players, most notably those who were in the NFL.
On July 25, the Journal of the American Medical Association published a study that examined the brains of 111 former NFL players who donated their brains to the Boston University CTE Center, reports the Boston Globe.
Of the 111 brains, 110 were diagnosed with CTE.
Dr. Ann McKee, director of the Boston University CTE Center, who co-authored the study, also examined Hernandez's brain. In a statement she said it showed "classic features of CTE," reports The New York Times.
A lawyer for his family called it "the most severe case they had ever seen in someone of Aaron's age."
Hernandez's estate filed a federal lawsuit on Sept. 21 against the NFL and the New England Patriots.
The suit alleges the league and the team knew that repeated blows to the head could lead to brain disease, but did not do enough to protect Hernandez.
The estate is also considering suing the NCAA and the University of Florida, where Hernandez played before being drafted by the Patriots.
Hernandez was arrested in June 2013 and charged with the murder of semi-pro football player Odin Lloyd, according to the Daily Mail. In April 2015, he was convicted of first-degree murder, and sentenced to life in prison.
While in prison, Hernandez was charged in a 2012 murder case in which two people were killed in a drive-by shooting, but was acquitted of both charges in April.
The day after he was acquitted, he hanged himself in his prison cell. He was 27.
The researchers who examined Hernandez's brain did not make a direct link between his violent behavior and his disease, according to The New York Times.
Sources: The New York Times, The Boston Globe, Boston University CTE Center / Featured Image: Jack Newton/Wikimedia Commons / Embedded Images: Steven Senne/The New York Times, Boston University School of Medicine via The New York Times