Ex-National Football League star Aaron Hernandez, who committed suicide in prison, suffered from an advanced stage of degenerative brain disease, researchers said on Thursday.

The former tight end for the New England Patriots had chronic traumatic encephalopathy (CTE), a type of neurological disorder that has been found in numerous dead ex-NFL players, Boston University's School of Medicine said in a statement. Hernandez had been serving a life sentence for murder when he hanged himself in his cell in April of this year.

Dr. Ann McKee, the director of the school's CTE Center, said Hernandez had stage three CTE, with stage four being the most severe form of the disease. The illness is believed to be caused by repeated blows to the head.

US media reported that Hernandez's lawyer, Jose Baez, has filed a lawsuit in an American court against the NFL and the Patriots on behalf of Hernandez's daughter and Shayanna Jenkins-Hernandez, who was engaged to him at the time of his death.

The lawsuit said Hernandez had "the most severe case of Chronic Traumatic Encephalopathy (CTE) medically seen in a person of his young age of 28 years" by Boston University researchers.

Hernandez signed a seven-year, $40 million contract with the Patriots prior to the 2012 season.

Hernandez, who played three seasons with the team, had had numerous run-ins with the law during his college football career at the University of Florida.

He was sentenced to life in prison in April 2015 for the murder of Odin Lloyd, a semi-pro football player whose bloodied body was found less than a mile from Hernandez's luxury Massachusetts home in 2013. The two men had been dating sisters.

Neuropathologists and mental health doctors have been studying the effects of repeated blows to the heads of NFL players for several years and the results are disturbing.

A previous study by McKee that was published this year in The Journal of the American Medical Association concluded that of 111 brains of deceased NFL players that were examined, 110 were found to have CTE.

Football linemen make up the biggest portion of those tested by McKee, partly because nearly half of the 22 players on the field are offensive and defensive linemen.

Linemen bang helmets on most plays and the general consensus is that the accumulation of seemingly benign, repeated blows probably causes CTE more than helmet-rattling concussion hits.

- Brick wall -

Researchers say that in a typical game the force on the player's head is equivalent to what you would see if he had driven his car into a brick wall at 48 kilometres per hour (30 mph).

Suicide is not uncommon among players who suffer the effects of CTE, but mental health experts caution that the jury is still out on whether there is a firm link between suicide and CTE.

Junior Seau, 43, whose brain was examined by neuropathologists, killed himself with a gunshot to his chest in May 2012.

Besides Seau, other players with CTE who have committed suicide include Terry Luther, Andre Waters, Shane Dronett, David Russell and Ray Easterling.