On a balmy Friday evening in the Dallas suburbs, Stephanie Richards stood outside the $60 million football stadium at Allen High School before the team’s game against McKinney High, its rival.

A lifelong Texan, she loves the spirit of Friday night lights. Her son, a sixth grader, has expressed interest in playing football, too, but wary of the risks of brain injury, she and her spouse have decided to hold off letting him play until he reaches high school.

Still, she said, there’s risk in any sport, and she feels reassured that the protective measures in place at the high school level — as opposed to youth football, which is staffed mostly with volunteer coaches — will keep him safe. “Do I think it’s a concern for him? No,” Ms. Richards said. “But I think as a parent, you’re always concerned about your kid.”

Like countless other parents, Ms. Richards has been trying to balance her love of football with her lingering worries about the risks of letting their son play such a violent game. For nearly two decades, she and other parents have heard about how concussions and years of playing a collision sport have damaged the brains of young and older players alike.

That delicate calculus between comfort and uneasiness was altered again by sobering news on Friday: Shane Tamura, who killed four people and himself in Midtown Manhattan in July, had chronic traumatic encephalopathy, the degenerative brain disease linked to repeated head hits sustained in football and other contact sports.

Known as C.T.E., the disease can be diagnosed only posthumously. It has been found primarily in older athletes years after they stopped playing, though a smaller but growing number of younger athletes — even those who don’t play beyond high school — have been found with the disease. Mr. Tamura was 27 and played football only through that level.

Researchers stress that much is unknown about the disease, including whether genetics or other factors play a role. There is no way to determine whether C.T.E., whose symptoms include mood swings, an inability to control aggression and impulses, and some degree of dementia, caused Mr. Tamura to go on a shooting spree, they add.

But Mr. Tamura’s diagnosis revived questions about the possible long-term dangers of playing tackle football, even at only the youth level. At many high school stadiums around the country, parents were reflecting on their decisions.

“I guess for a long time, most people like myself probably took it for granted, not knowing what the effects were,” said Cleveland Baker, 54, of Brown Deer, Wis., who on Friday watched his younger son compete against University School of Milwaukee.

Now, the dangers of football are definitely front of mind for the Baker family. Mr. Baker’s older son, Jabari, showed promise as a two-way lineman at Brown Deer High School during his junior year. But after he suffered two concussions, Mr. Baker and his wife gave him an ultimatum: If he had a third concussion, his football career would be over.

“Between my wife and I, we probably would have said no” to his returning, Mr. Baker said. “But he was so passionate about the game. At that point, he wanted to see how far he could go.”

That son made it through his senior year. But watching his younger son play linebacker and special teams was “scary,” Mr. Baker said. The couple are making sure they monitor their sons and stressed that players’ cognitive and mental health needs to receive the same attention as their physical health.

More than a decade has passed since researchers began finding correlations between C.T.E. and boys who began playing tackle football before the age of 12 and, in a separate study, that the risks of C.T.E. increased the longer people played the sport. These and other findings, amplified by the deaths of hundreds of N.F.L. players who were found to have C.T.E., led to a steep decline in participation in youth tackle football between 2015 and 2020, according to the Sports and Fitness Industry Association.

One reason participation has rebounded since then is because leaders at every level of football, from Pop Warner to the N.F.L., have tried to address worries by teaching tackling techniques to reduce head hits, treating concussions more proactively and introducing new equipment designed to reduce the impact of blows to the head. Leagues have also promoted flag football as a safer alternative.

Many parents, though, are establishing boundaries. In Allen, Texas, Johnny and Linda Nuño understood that C.T.E. was associated with the accumulation of thousands of small, sub-concussive hits, not necessarily recognizable concussions.

“These head injuries don’t start at the N.F.L. — they start in pee wee leagues, then middle school, high school, college,” Mr. Nuño said. “By the time you get to the N.F.L., you’ve had some injury.”

The Nuños also understood that football has a hold on boys, including their 13-year-old son, Nicolas Blake. So they let him play football as a seventh grader, but they worried all season.

“An impact is an impact,” Mr. Nuño said.

“I was so happy when he was on the bench,” Mrs. Nuño added.

At the end of the season, the Nuños gently steered their son toward basketball and theater.

Some parents are going further. At Don Bosco Preparatory High School in Ramsey, N.J., where the second-best team in the state faced off against top-ranked Bergen Catholic High School, Robert Mosley, 55, watched with conflicted emotions.

A 1988 graduate of Don Bosco, he played wide receiver and joined the long football tradition at the school. But as he read about the risk of long-term damage from the game, he decided he didn’t want his son to play.

“God forbid he got brain damage or anything like that,” Mr. Mosley said. “I didn’t want that for him.”

Not surprisingly, adults at high school football games can be more inclined to support the sport and perhaps minimize its risks. Joseph Williams, 50, played football at Greenville High School in Greenville, Miss., and he returned on Friday to watch his old team. He has two daughters, so he never had to decide whether the game was safe enough for a son.

But he recognized the dangers of playing football, and was not surprised that Mr. Tamura was found to have C.T.E.

“The way I am thinking about this is, football is a contact sport,” he said. “So you have to expect there are risks to playing.”

In Orlando, Shayne Robinson, 42, who was working at a food truck and left it to head to Jones High School to attend a football game, said he didn’t think much about head injuries when he played tackle football in the early 2000s. But he does now.

“I think there should be more precautions used: rules, regulations and also equipment needs to be highly upgraded to protect the guys from C.T.E.,” he said. “These are human beings that we’re talking about.” He added that people needed to focus just as much on the health of players as they did on the money made from football.

“Let’s care about them the same way, because there’s life after football,” he said.

Audra D. S. Burch contributed reporting from Greenville, Miss.