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When Junior Seau was hitting people for a living in the NFL, he wore No. 55 and weighed 250 pounds. At the National Institutes of Health, his case number was SS-3590 and his brain weighed 1,580 grams.

Word came Thursday that Seau had a degenerative brain disease when he shot himself in the chest last May. Most shocking was that it was hardly a shock at all. His is merely the latest of dozens of cases of former pro football players who died with signs of chronic traumatic encephalopathy (CTE) and the third by suicide in recent times.

MORE: Seau's death serves as reality check

But Seau, for 20 seasons one of the NFL's most feared linebackers, is perhaps the most famous victim to date. His family donated his brain to NIH's National Institute of Neurological Disorders and Stroke last June, about six weeks after a girlfriend found him dead in his home in Oceanside, Calif.

A six-month study was completed a week before Christmas and released Thursday with his family's blessing.

"On initial examination the brain looked normal but under the microscope, with the use of special staining techniques, abnormalities were found that were consistent" with a form of CTE, NIH said in a statement. It added that a small region of Seau's left frontal lobe showed scarring consistent with a small, old, traumatic brain injury.

"In some ways, I'm relieved there's this finding," Mark Walczak told USA TODAY Sports. He is a former teammate and friend who spent time with Seau in the last week of his life.

"I'm not surprised that Junior and others have this evidence of brain damage," Walczak said. "He's not the only one that has these types of issues. We've got guys who are retiring as we speak, like Ray Lewis, that we need to keep an eye on."

Tavares Gooden, San Francisco 49ers linebacker and veteran of five NFL seasons, hadn't heard the news about Seau. He said that when he puts on his helmet each day he's not thinking about potential long-term effects from hits to the head.

However, he knows enough to monitor himself.

"We can all learn something from Junior," Gooden told USA TODAY Sports on Thursday. "I've been getting CAT scans to make sure things are right in my brain. Every year I get a check-up. … I try to stay on top of things."

Officially, Seau never suffered a concussion during a two-decade career with the San Diego Chargers, Miami Dolphins and New England Patriots that ended with his 2009 retirement. But Walczak, a former tight end and long snapper in the NFL, believes his friend suffered multiple undiagnosed concussions.

"Junior just didn't report head injuries," Walczak said. "I had (unreported) concussions, too, especially back when guys were allowed to tee off on the long snappers. But you just don't report them. You're a football player. You're tough. If you did report stuff like that, next thing you know you're on waivers."

The NFL, in a statement to USA TODAY Sports, said in part: "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."

The statement said the NFL had earlier committed a $30 million research grant to NIH and is working with the NFL Players Association on investment of an additional $100 million for medical research, per the collective bargaining agreement.

"We have work to do," the statement said, "and we're doing it."

Walczak expressed skepticism about the NFL's efforts.

"I think they've basically just scratched the surface of a gigantic iceberg," he said. "There's so much more to do for the players from the past and the players who are playing now. They need to take some more steps to rectify things. They need to acknowledge the fact that the players who have played in the past have suffered. They need to try to eliminate more of these tragedies.

"They've got to facilitate something for the retired players. They turned a blind eye to this for so long, and now it's an avalanche."

Russell Lonser, chair of Ohio State's Wexner Medical Center Department of Neurological Surgery, oversaw the NIH study.

"There's still a lot to learn about CTE," Lonser told USA TODAY Sports. "We don't have a basic understanding of the incidents and prevalence. From a research perspective, what needs to be done" is long-term studies.

That won't be of much solace to those playing now and those who have retired.

"I understand it won't be for those individuals," said Lonser, who is head of the NFL's research subcommittee and a member of the NFL's head, neck and spine medical committee. "But (long-term research) is critical to our understanding it. I think it goes back to many disorders that occur: The first step is a better understanding."

The trouble is that CTE cannot be diagnosed until after death. Lonser believes the day will come when it can be diagnosed in the living, which "would give us ways, potentially, to treat it. That's the ultimate hope."

Lonser, former chief of surgical neurology at NIH, said three experts independently arrived at the same conclusion in Seau's case. "The three experts from outside the federal government reviewed it with other brain samples and were not told that one of those three samples they looked at was Junior Seau's," Lonser said.

Chicago neurosurgeon Julian Bailes of NorthShore University Health System is director of the Brain Injury Research Institute. He told USA TODAY Sports that the Seau finding supports his belief that CTE can be caused by the cumulative effects of football head impacts that don't result in formally diagnosed concussions.

"When (Seau) died, I said if there was anybody you'd worry about having CTE it was certainly someone who played 30 years, including 20 at the NFL level," Bailes said. "I remember Mike Webster, the first case, played 17 years in the NFL."

In 2002 in Pittsburgh, Bailes and neuropathologist Bennet Omalu identified Webster, a deceased former star center with the Steelers, as the first NFL case of CTE, once associated primarily with boxers. Bailes said Webster, like Seau, had never been diagnosed with a concussion during his pro career.

Bailes said a key area of research is whether some players might have a genetic predisposition to CTE, which could explain why some get the disease while others don't.

"There's likely a genetic predisposition, just like there is to everything else in life," Bailes said. "But I do think that it's probably going to be shown that it's on an exposure (to head contact) basis. If anybody played a long time, it was (Seau). He played youth football, he played high school, college and 20 years in the NFL."

Omalu and Bailes also found CTE in the autopsied brain of former Cincinnati Bengals wide receiver Chris Henry, who was killed in North Carolina in 2009 when, according to police, he fell off the back of a pickup truck driven by his fiancé during a domestic dispute.

"Chris Henry was the youngest and the only active NFL player ever diagnosed with CTE. … He was 26. No concussion history," Bailes said.

Bailes favors expanded efforts to take head contact out of football. He would like to see linemen start off each play in a stand-up position instead of a three-point stance to eliminate the head-to-head contact that starts every play

As medical director of Pop Warner youth football, Bailes was involved this year in new rules limiting head contact in youth practices. He wants stringent enforcement of rules to protect the head.

"I've been saying for several years that we've got to get the head out of the game," Bailes said.

ABC News/ESPN broke the Seau story Thursday on Good Morning America. Gina Seau, his former wife, said the family was told the disease came from "a lot of head-to-head collisions over the course of 20 years of playing in the NFL. And that it gradually, you know, developed the deterioration of his brain and his ability to think logically."

Asked if she believed the NFL was slow to address the issue, she said, "Too slow for us, yeah."

Seau's name joined a list of several dozen deceased football players identified as having CTE. Boston University's center for study of the disease reported last month that 34 former pro players and nine who played only college football suffered from CTE.

Besides Webster and Henry, other cases of CTE in former NFL players identified by Bailes' Brain Injury Research Institute include Terry Long and Justin Strzelczyk of the Pittsburgh Steelers and Andre Waters of the Philadelphia Eagles.

The NFL is facing a federal lawsuit representing more than 4,000 former players claiming the league refused to acknowledge the link between brain damage and the sport, even after CTE was found in former players. The first suit was filed in August 2011. Now, about 190 such suits have been consolidated in federal court in Philadelphia. The Seau family has not decided about joining the lawsuits.

"I'm not shocked by the news. I know how Junior played. He played hard for 20 years," said ESPN analyst Lomas Brown, a former NFL offensive lineman who is part of the lawsuit. "This is a real condition, CTE. It's not just a group of retired guys trying to get money."

ESPN analyst Christian Fauria, a former NFL tight end, said he still doesn't believe enough to join the lawsuit.

"I just don't feel right signing up for it. I don't," he said. "I forget a guy's name I knew two minutes ago. 'Do I need to join this (lawsuit)? Is it just my age catching up to me?' I don't know how many concussions I had that were a product of pro football, or of high school football, of fighting my brother tooth and nail for 15 years. ...

"For me to jump into the lawsuit based on everything I see, and questioning the motives of some players — I'll probably get screamed at for this — but some players, I don't know what their motives are."

The other former players who killed themselves and were found to have evidence of CTE are former Chicago Bears defensive back Dave Duerson and former Atlanta Falcons safety Ray Easterling.

Duerson left a note asking for his brain to be studied for signs of trauma before shooting himself. His family filed a wrongful death suit against the NFL, claiming the league didn't do enough to prevent or treat the concussions that severely damaged his brain.

Easterling suffered from dementia, depression and insomnia after his career, according to his wife, Mary Ann. He committed suicide last April. She is among the plaintiffs who have sued the NFL

With approval from the families, the Boston University study identified three of the former players diagnosed with CTE: John Mackey, who died last year at 69 and played 10 NFL seasons as a tight end, all but one with the Baltimore Colts; Ollie Matson, who died last year at 80 and played 14 NFL seasons as a running back with the Chicago Cardinals, Los Angeles Rams, Detroit Lions and Eagles; and Cookie Gilchrist, who died in 2011 at 75 and played for CFL teams and for the AFL's Buffalo Bills, Denver Broncos and Dolphins.

That study was done by investigators from Boston University's Center for the Study of Traumatic Encephalopathy and the Veterans Affairs Boston Healthcare System, in collaboration with the Boston-based Sports Legacy Institute — a group focused on brain injury in athletes and other high-risk groups, such as the military.

The study also reported early-stage cases of CTE among some individuals who only played football through high school. The investigators said that was further cause for alarm about head injuries in high school and youth football and other youth sports with head impacts.

That study identified one of the former high school players as Eric Pelly, who died in suburban Pittsburgh at 18 in 2006. Researchers said he had early stage CTE. Pelly sustained concussions as a high school football and amateur rugby player. He died 10 days after sustaining a concussion in a rugby game. He had a concussion two years earlier while playing high school football.

Cultural critics emerged Thursday with the Seau news. Orin Starn, professor of cultural anthropology at Duke, said in a release: "Americans don't seem to care that NFL players put their lives at risk every Sunday. … The NFL has become America's favorite blood sport, a modern version of the ancient Roman coliseum where thousands gathered to cheer brutal gladiatorial combat."

Ta-Nehisi Coates, a senior editor at The Atlantic, wrote in an online column: "You can't fix this by getting rid of big hits. You can't fix this by focusing on concussions. Junior Seau never had such a diagnosis, and even if he did, it is the repeated 'minor' hits that cause CTE. The enemy is the game itself. And it is killing men."

Reaction came, too, from NFL locker rooms.

"It's scary," Seattle Seahawks Pro Bowl center Max Unger told USA TODAY Sports. "Football will always and forever be a sport with pretty severe consequences, at least at the professional level. We're talking about 10-, 11-, 12-year careers. Everybody knows the risks that are involved. I don't think that's really surprising.

"It's a tradeoff, though. You make a pretty big salary in a relatively short period of time. The tradeoff is you get beat up a little bit. I think the (get-back-in-there) culture is changing a little bit. But it's scary. That's the reality we live in. I don't know there's a lot you can do."

The 49ers' Gooden says as long as he's playing, he can't let what might happen down the line impact him.

"You can't do that," he said. "You can only live for right now. That's all you can do because we all know what we signed up for. I signed up for this game when I was 10 years old, and I knew the consequences of it.

"Nobody thinks about the bad side. You only think about the good side, and that's what everybody does. You leave the rest to God and you pray and hope that nothing happens to you.''

Gooden said he has had three or four concussions: "I had one in college at the University of Miami, and I think I had three or two in Baltimore" with the Ravens.

Is he doing okay?

"I am as of today," he said. "There are some things that go on where I can't look into the light as well as I want to. I have light sensitivity, and sometimes I get migraine headaches due to the concussions. … My mood's not at that level where I don't want to live or anything like that."