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Aaron Hernandez's fiancee, Shayanna Jenkins-Hernandez, says Aaron killed himself as a result of severe CTE he suffered during his NFL career -- and she's suing the the Patriots and the league for keeping him in the dark about the brain disease.

Shayanna filed the lawsuit in Mass. on behalf of the daughter she had with Aaron.

In the suit Shayanna says, "Aaron had stage 3 CTE usually seen in players with a median age of death of 67 years."

Shayanna claims the NFL and the Patriots were aware of the link between suicidal impulses and CTE and failed to share that info with Hernandez.

She claims the NFL and the Patriots "were fully aware of the damage that could be inflicted from repetitive impact injuries and failed to disclose, treat or protect him from the dangers of such damage."

In her suit, Shayanna says she and her daughter are seeking "redress for the loss of parental consortium she has experienced based on the negligent conduct of Defendants that deprived her of the companionship and society of her father, Aaron Hernandez."

Hernandez infamously killed himself in his prison cell on April 19 -- after he was found not guilty in a double homicide. Hernandez was already serving a life sentence for the murder of Odin Lloyd at the time of his death.

Shayanna says in her suit, "On April 19, 2017, Aaron succumbed to the symptoms of CTE and committed suicide."