Junior Seau, one of the NFL's best and fiercest players for nearly two decades, had a degenerative brain disease when he committed suicide last May, the National Institutes of Health told The Associated Press on Thursday.

Results of an NIH study of Seau's brain revealed abnormalities consistent with chronic traumatic encephalopathy (CTE).

"The brain was independently evaluated by multiple experts, in a blind fashion," said Dr. Russell Lonser, who oversaw the study. "We had the opportunity to get multiple experts involved in a way they wouldn't be able to directly identify his tissue even if they knew he was one of the individuals studied."

The NIH, based in Bethesda, Md., conducted a study of three unidentified brains, one of which was Seau's. It said the findings on Seau were similar to autopsies of people "with exposure to repetitive head injuries."

Seau's family requested the analysis of his brain.

Seau was a star linebacker for 20 NFL seasons with San Diego, Miami and New England before retiring in 2009. He died of a self-inflicted shotgun wound.

He joins a list of several dozen football players who had CTE. Boston University's centre for study of the disease reported last month that 34 former pro players and nine who played only college football suffered from CTE.

"I was not surprised after learning a little about CTE that he had it," Seau's 23-year-old son Tyler said. "He did play so many years at that level. I was more just kind of angry I didn't do something more and have the awareness to help him more, and now it is too late.

"I don't think any of us were aware of the side effects that could be going on with head trauma until he passed away. We didn't know his behaviour was from head trauma."

That behaviour, according to Tyler Seau and Junior's ex-wife Gina, included wild mood swings, irrationality, forgetfulness, insomnia and depression.

"He emotionally detached himself and would kind of 'go away' for a little bit," Tyler Seau said. "And then the depression and things like that. It started to progressively get worse."

He hid it well in public, they said. But not when he was with family or close friends.

"We appreciate the Seau family's co-operation with the National Institutes of Health," the league said in an email to the AP. "The finding underscores the recognized need for additional research to accelerate a fuller understanding of CTE.