Former NFL player Aaron Hernandez's degenerative brain disease "undoubtedly took years to develop", researchers have said.
Hernandez was 27 when he hanged himself in prison while serving a life sentence for the murder of his friend Odin Lloyd in 2013.
Days before his death, he was acquitted in the 2012 drive-by shootings of two men in Boston.
Former NFL star Aaron Hernandez had CTE
In September, Hernandez's lawyer Jose Beaz said the former New England Patriots star had the most severe case of chronic traumatic encephalopathy (CTE) that neurologists have ever discovered in someone so young.
Now researchers have revealed the full extent of Hernandez's brain damage.
Dr Ann McKee, director of Boston University's CTE Center, said the outside of the brain seemed normal but the inside was riddled with evidence of CTE and small haemorrhages associated with head trauma.
The hippocampus - vital for memory - had started to shrink; and the frontal lobe, which affects impulse control, was also badly damaged.
Such extensive damage from the disease had not been seen in anyone younger than 46 years old before, Dr McKee said.
She added: "We can say collectively, in our collective experience, that individuals with CTE and CTE of this severity have difficulty with impulse control, decision-making, inhibition of impulses or aggression, often emotional volatility and rage behaviours.
"These are very unusual findings to see in an individual of this age," McKee said. "We've never seen this in our 468 brains, except in individuals some 20 years older.
She added: "We don't get as many specimens from this age group and in this age group, he's clearly at the severe end of the spectrum.
"(This is) substantial damage that undoubtedly took years to develop.
"I imagine these changes have been evolving over maybe even as long as a decade."
Dr McKee, who has studied hundreds of donated brains over more than a decade, insisted that she could not link the disease to Hernandez's behaviour, however.
It was confirmed in September that he had stage three (out of four) of the disease, which is linked to repeated head trauma and can only be diagnosed through a post-mortem examination.
It has also affected former soldiers and players of contact sports, such as boxing and football.
Dr McKee said: "I think there is a concern that we're seeing accelerated disease in young athletes.
"Whether or not that's because they're playing more aggressively or if they're starting at younger ages, we don't know, but we're seeing the ravages of this disease and certainly he's an example in a young person."