(CNN) -- Star NFL linebacker Junior Seau -- just 43 years old when he took his own life last May -- suffered from chronic traumatic encephalopathy, a neurodegenerative brain disease that can follow multiple hits to the head, the National Institutes of Health said Thursday.

Questions of CTE came up immediately after Seau's body was found, with a handgun nearby, in the bedroom of his home in Oceanside, California.

CTE, a progressive neuro-degenerative disease, can result in Alzheimer's-like symptoms such as dementia, memory loss, aggression and depression, but it can be diagnosed only after death.

Seau's family donated his brain to the National Institutes of Health for research, and Thursday the NIH released a statement saying "abnormalities were found that are consistent with a form of chronic traumatic encephalopathy (CTE)."

According to the pathology report from the NIH, five researchers -- two NIH neuropathologists and three independent experts -- examined slides of Seau's brain and all confirmed that there were signs consistent with CTE. None of the researchers was aware of the identity of the brain when initially looking at it.

Dr. Ann McKee, the director of neuropathology at VA Boston, who was not involved in this case, has looked at CTE in most of the NFL players' brains studied so far. In a recent study she co-authored, the researchers found CTE in the brains of 34 of 35 NFL players. She says the CTE diagnosis in the Seau case was not unexpected.

"From what I've read about the symptoms (Seau) was experiencing the last couple of years -- the ones relayed by the family -- it is not surprising to me that he had this disease," McKee said. "It doesn't sound like it was early CTE, that it was becoming quite widespread in the brain. And he was young at the time of death. It is another sad day to see another fairly well-established case of CTE."

What may be a surprise to some is that Seau was never diagnosed with a concussion in all the decades he played football. That points to the bigger mysteries of the disease that scientists such as Dr. Julian Bailes, the co-director of the NorthShore Neurological Institute, hope to someday solve.

"It is not unprecedented that he didn't have a concussion history. That's part of the problem in figuring this out," Bailes said. "It seemed logical at first that it would be people with multiple concussions that would be at risk of CTE. As we've learned more, it was surprising that some of those at risk for CTE were players who did not have a history of concussion."

Seau was one of a string of high-profile NFL players -- along with Dave Duerson, Shane Dronett and Shane Easterling -- who tragically took their lives and were later diagnosed with CTE.

Not everyone who is exposed to repeated head trauma would develop the disease, experts say.

"Based on what we know thus far, I think we have to assume that the number one risk factor we have is the degree or extent of exposure," Bailes said. "And if anyone had high exposure, if there was anyone you'd worry about, it'd be Junior Seau. He played for 30 years. He played youth football, college football, for 20 years in the NFL."

The National Football League responded to the announcement of Seau's diagnosis with a statement saying, "We appreciate the Seau family's cooperation with the National Institutes of Health. The finding underscores the recognized need for additional research to accelerate a fuller understanding of CTE."