One of the most celebrated quarterbacks of all time had been suffering from a degenerative brain disease linked with repeated head trauma, a report claims.
Before his death from colon cancer at 69 in July, Ken Stabler requested his brain be donated to research into chronic traumatic encephalopathy (CTE).
The New York Times reports Stabler was found to have a "quite severe" case.
Stabler was the NFL's Most Valuable Player in 1974 and led the Oakland Raiders to the Super Bowl in 1977.
He threw for 194 career touchdowns and 27,938 yards over 15 seasons and was a four-time Pro Bowl selection.
Some 5,000 former players have sued the NFL, claiming it hid the dangers of repeated head trauma.
CTE is associated with symptoms such as memory loss, depression and progressive dementia.
The disease, which can only be diagnosed after death, has been found in the brains of dozens of former American football players.
The former players suing the NFL agreed to a settlement which is under appeal, but could cost the league $1bn (£685m).