LIVERMORE — George Atkinson had had enough. The famed Raiders alumnus had been talking for almost an hour when the mood suddenly soured.

His eyes drooped, head bowed. The voice slightly slurred. The once-invincible All-Pro defensive back slowly rose from a chair, then disappeared for a while into another room.

“You see that?” he asked later. “My mood just swung.”

Irritability. Depression. Short-term memory loss. Atkinson, 69, suffers from symptoms that someday might be attributed to chronic traumatic encephalopathy, a degenerative brain condition associated with a growing list of former NFL players and thought to be the result of repeated blows to the head.

The anxiety over football and concussions lingers in Atkinson’s split-level home on a quiet, manicured street near Livermore’s vineyards. This is where a player once known for bloodcurdling hits grapples with the prospect of a deteriorating mind while supporting his twins’ journeys to the NFL.

A fierce man who once held a contest with teammate Jack Tatum over who got the most knockouts in a game doesn’t know what to expect anymore, which partly explains why he decided to pledge his brain to Boston’s Concussion Legacy Foundation for CTE research.

He is among an increasing number of players determined to help researchers rather than bury their head in the sand. Meanwhile, a game symbolizing American toughness and independence is fighting for its dignity in the face of mushrooming concern that playing football from youth through the NFL is leaving participants such as Atkinson struggling to deal with life in their golden years.

“When you have sons do what they want to do, you want them to understand what they are buying into and what could happen — what’s more than likely going to happen,” Atkinson said.

‘A collision sport’

The potential outcome scares Josh Atkinson, who wore No. 43 in high school and college to honor the man he calls “Pops.” But it doesn’t stunt his dream to play professionally as the defensive back prepares for the NFL draft April 28.

“They played the game a lot different back in my pop’s days,” he said. “When you know better you do better.”

George Atkinson and Denise Fee support their kids. Running back George III has been with the Raiders practice squad since 2014 and is hoping for a breakthrough next season.

“It’s not ours to take away,” Fee said. “It just runs through their veins so that they can’t stop — they’ve got to play.”

Atkinson helped coach the twins at Granada High in Livermore just as he started noticing his symptoms. But he remained emphatic that football was their own choice to make as they headed off to Notre Dame.

“I would never tell them not to,” said Atkinson, who said it is safer now with rules and equipment upgrades. “They know what the risks are. It’s not like they are walking into it blindly.”

The same couldn’t be said for Atkinson, who grew up in football-loving Georgia and played at Morris Brown College of Atlanta.

“The only way you didn’t get your bell rung was not making contact,” said Atkinson, who played some games without remembering anything about them.

Back in the outlaw days, Oakland created a menacing image that the 6-foot-1, 185-pound Atkinson helped perpetuate on his way to becoming rookie of the year in 1968. He developed a punishing drag-down-by-the-head tackle teammates called the Hook.

Atkinson cemented his reputation for causing bedlam by delivering two ghoulish concussions to Pittsburgh star receiver Lynn Swann.

The first occurred in the 1975 American Football Conference championship against the Steelers. Then it happened again in the ’76 season opener when Atkinson slammed a forearm into Swann’s head on a play away from the ball that was still captured on replay. Pittsburgh’s Chuck Noll referred to Atkinson as part of football’s “criminal element,” which led to an unsuccessful $2 million defamation suit against the coach and the Steelers.

“Football is a collision sport,” Atkinson said. “If you don’t have a certain mentality you will get run out of the game.”

Swann, who once called Atkinson a “headhunter,” said he suffered from tunnel vision and severe headaches from those concussions.

“Sure, intimidation is part of the game, … but you don’t go for the head and try to injure him,” the former receiver from San Mateo once said. “How can anybody call that ‘sport’?”

Atkinson is unapologetic, saying an explosive tackling technique was needed to upend guys bigger than him.

“I don’t wish what is happening to me on anyone else at all,” he said.

But a player who was going to be part of that Raiders team that won Super Bowl XI was not going to be thinking about the consequences of those violent hits, Atkinson said.

“I have to turn inward to me 100 percent just to try to stay (on an) even keel,” he said. “That’s the job for me.”

Addressing the issue

Atkinson came to understand his slow decline might be serious while working as a Raiders broadcaster in the mid-2000s when he’d forget what he planned to say on air.

“I noticed it before he did,” Fee said. “I didn’t know what it was.”

Fee nudged Atkinson to see a neurologist six years ago. He now gets checked every three months and also practices brain games recommended by a neuropsychologist.

“It’s sad and depressing all in one,” said Tom Flores, the former Raiders quarterback and coach who worked with Atkinson on the team’s broadcasts. “You’re depressed not only for George, but for some of my other friends.”

Flores, 79, hasn’t been around Atkinson enough to notice the changes because he lives in Indian Wells. Jim Plunkett, another Raiders star quarterback who was part of the broadcasting team with Atkinson, also hasn’t observed any of the symptoms.

“George is the same goofy guy I’ve known for 40 years,” Plunkett said.

But both former Raiders are acutely aware of one of football’s most important issues.

Flores, for one, is conflicted. He can recall suffering at least 10 concussions from high school to the pros and has sought alternative treatment since 2009 with hyperbaric oxygen. At the same time, the Super Bowl-winning coach said the NFL doesn’t deserve all the blame.

“They’re scaring the hell out of people talking about things they really don’t know,” said Flores, who said his brain activity has increased because of the treatments. “Some of it is because of football — some of it is because we’re getting older. You don’t want to make everybody a charity case.”

Plunkett, 68, hasn’t seen a neurologist because the only symptom he has suffered is forgetfulness.

“The thing that scares me more than anything else is not remembering my son and grandson,” Stanford’s only Heisman Trophy winner said.

But Plunkett supports the attention the concussion issue is getting for all the players who have experienced serious symptoms that could point to CTE.

“I don’t know what the truth is — whether they tried to cover it up — but the league needs to come forward more as well,” he said.

After the death of Raiders quarterback Ken Stabler, Atkinson joined some former teammates to donate their brains. Stabler’s is among 90 of 94 NFL players’ brains examined at the Boston University School of Medicine that show CTE.

When recently announcing the pledge to this news organization, Atkinson and Fee met with Chris Nowinski, co-founder of the Concussion Legacy Foundation and a doctoral candidate in behavioral neuroscience at Boston University.

They peppered the former professional wrestler with questions about advances in brain science.

“You can spark the disease while you’re playing, and it can be only a little bit of a problem,” Nowinski told them. “Then the rest of your life it eats away at the brain.”

Atkinson and Fee nodded in agreement.

Fee has found a disoriented Atkinson sitting on the side of the bed gazing out in the blackness at night. They sleep with the television on because she said it comforts Atkinson.

Sometimes, he retreats into the home for days at a time in the throes of depression. “I know when I wake up in the morning in what direction I’m headed, by that depressed feeling or feeling normal,” Atkinson said.

The family, which includes 11 children, counting two stepdaughters, has rallied around the patriarch with good cheer and hope while sidestepping feelings of despair about what the future might hold.

Fee notices the brightness returns to the eyes whenever he’s with the kids and grandkids. They approach the changing moods with tenderness and help give Atkinson a sense of continuity.

“It doesn’t matter where he goes with his mind,” Fee said. “I’m not going anywhere.”

Contact Elliott Almond at 408-920-5865, and follow him at Twitter.com/elliottalmond.