That answer sidesteps the key point. There is much unknown about brain trauma and C.T.E., including the role of individual body chemistry and the precise nature of the most dangerous hits. Those brains studied, by definition, came from men who worried about brain degeneration. Still it appears irrefutable that a link exists, and that nutrition and better ways of tackling and blocking will most likely not change that.

No freshman player, Rueda said, ever declined to sign a health waiver.

I sought the counsel of Brian Cabral on the question of better training. He was a magnificent linebacker at the University of Colorado and played nine years in the N.F.L., earning a Super Bowl ring with the Chicago Bears. Then he returned to Colorado and became a linebacker coach.

He recruited the running back Salaam and coached the linebacker Wahlroos, both of whom are dead. He also coached Ted Johnson, a brilliant linebacker who has spoken with aching honesty of his struggles with depression and memory loss after more concussions than he can count.

Cabral loves the band of brothers aspect of football, and yet he cannot dodge the shadows. He knows too much, and several times during our conversation he paused, choked up.

“I hate to say this, but I taught players what I was taught,” he said.

And what was that technique?

“Put your helmet right in the guy’s jaw and drive up through his throat to his head,” he said. “I regret it, I really do.”

I noted that officials said better technique might offer a sort of salvation. He nodded yes, and then quickly shook his head. “They try to take the head out of tackling, but come on,” he said. “We bang heads and guys get concussions.”