DUBLIN, Ga. — Demaryius Thomas’s parents see their son every day.

A painting of the former N.F.L. star rests against a wall in Katina Smith’s home, and Bobby Thomas, his father, keeps the same image on his cellphone. It depicts a cherished moment that now seems foreboding: The two beaming parents flank their son in the moments after his Denver Broncos won Super Bowl 50 as Demaryius looks downward with a pained expression, scratching the back of his head.

The receiver had been leveled by Carolina Panthers linebacker Luke Kuechly during the game and had a headache so bad that he missed most of the parties after the victory.

“He was like, ‘Hey, y’all, I need to leave and go by myself because I don’t feel too good,’” Smith recounted. “And so, you know, he left and didn’t even finish celebrating or anything like that.”

Demaryius Thomas died in December at 33, mere months after retiring from a Pro-Bowl career in the N.F.L. in which his charisma, humility and team-first ethos on the field made him a favorite of teammates and fans. Those closest to him said his behavior became increasingly erratic in the last year of his life, which was marked by the memory loss, paranoia and isolation that are hallmarks of chronic traumatic encephalopathy, the degenerative brain disease linked to repeated head hits.

On Tuesday, doctors from Boston University announced that Thomas was posthumously diagnosed with Stage 2 C.T.E., but his life and death were also complicated by seizures brought on by a 2019 car crash. They attacked with little or no warning and led Thomas to wreck other cars and fall down steps. The coroner’s office in Fulton County, Ga., has not yet ruled on the cause of his death, but doctors in Boston said he most likely died after a seizure.

“He had two different conditions in parallel,” said Dr. Ann McKee, the neuropathologist who studied Thomas’s brain. She added that seizures were not generally associated with C.T.E.

Because of the dual conditions, Thomas’s C.T.E. diagnosis does not bring the neat clarity that has punctuated other N.F.L. players’ demises. His family, friends and former teammates will not know how much football is responsible for Thomas’s struggles and are only now coming to grips with the extent to which he suffered.

“It amazes me now when we talk about how a young man that age can be in so much pain but still smile,” said Carlos Jones, Thomas’s pastor who was with him when a seizure caused Thomas to fall down the steps in his home in early 2021. “It was just a testament of how strong he was.”

Reunited at the Super Bowl

Football changed the trajectory of Thomas’s life, his achievements on the field helping to stabilize his family that was fractured during his adolescence.

Thomas was born on Christmas Day, 1987, in Montrose, Ga., a speck of a town between Macon and Savannah. Katina was 15 when she gave birth to him, and she never married Bobby, who joined the army and was often away.

When Thomas was 11, federal agents burst into the family’s home with a search warrant and found money connected to a drug ring led by Smith’s mother, Minnie Pearl Thomas. They arrested Smith for conspiracy to distribute cocaine and after she refused to testify against her mother, she was sentenced to 20 years in prison. Minnie Pearl Thomas was given a life sentence.

Thomas bounced from home to home for a couple of years before settling with Bobby Thomas’s sister, Shirley, and her husband, James. Schoolmates bullied Thomas because his mother was in prison, but he found solace and affirmation in track, football and basketball. In sports, overcoming pain was a key to his success.

“He had a lot of injuries that he played through, and he would always say, ‘You know how I was raised, you know how I was trained, I’m not going to let my team down,’” said Paul Williams, Thomas’s high school basketball coach and close friend. He said Thomas always had a ready smile despite his many challenges off the field.

Denver drafted Thomas 22nd overall in 2010, the first receiver taken that year, and his career skyrocketed when quarterback Peyton Manning arrived in 2012, the first of five consecutive years in which he had 1,000 or more receiving yards. Thomas became a mentor to many teammates, including Bennie Fowler, a fellow receiver, and was a favorite teammate for his affable workmanlike approach to the game.

Denver reached the Super Bowl the next season and was trounced by the Seattle Seahawks, but Thomas’s 13 catches set what was then a record for receptions in the title game.

In the lead up to Thomas’s next championship appearance, his family history gained as much attention as his play. After 17 years of appeals and lobbying from the family, President Barack Obama commuted Smith’s sentence as part of a Department of Justice focus on clemency for nonviolent drug offenders. Their story became a focus of the lead up to Super Bowl 50, with media reporting extensively about Smith’s finally being able to watch her son play in person on the game’s biggest stage.

Thomas, who had been meeting with lawyers and who penned a letter to Obama on his mother’s behalf, was never happier.

“He loved her to death,” said Jamuel Jones, one of Thomas’s friends from high school. “I saw a spark in him when she got out. They talked every day. That was his main goal, to get them out,” he said, referring to Thomas’s mother and grandmother.

(Obama commuted Minnie Pearl Thomas’s sentence in 2016.)

‘It ain’t easy leaving football.’

As high as football lifted Thomas, it also contributed in some measure to his rapid decline. In the years after that high-water mark shown in the painting, Manning retired and Thomas’s injuries piled up. Smith said her son told her that his peripheral vision was diminished.

In 2019, Thomas had been driving 70 miles per hour in a 30-m.p.h. zone in Denver when he lost control and flipped his car multiple times. His head cracked the windshield and the Jaws of Life were needed to remove him from the vehicle. Jamuel Jones, who had also played college football, was riding in the passenger seat and said doctors told the two football players that their ability to absorb hits might have saved their lives.

Thomas played a final, desultory season with the Jets, then went home to Georgia, his life at a crossroads. He was not under contract and was unsure about playing during the pandemic, yet he was determined to get another 237 yards to reach 10,000 career receiving yards. So he worked out five days a week, but his comeback was stalled by seizures that began in the fall of 2020.

As the seizures grew in number and intensity, neurologists told him they might be related to stress. The anti-seizure medicine Thomas took made him sluggish, and a second prescription did not stop them, so he tried ozone therapy, a hyperbaric chamber, massages and other treatments that had little lasting impact.

“He spent a lot of money on his body and look what happened, you know?” said Bobby Thomas, who fell into a depression when Demaryius died that deepened as he learned of the severity of his son’s condition.

“I didn’t know that he was that bad off.”

In a video announcing his retirement last June, Demaryius Thomas acknowledged that he was trying to find his way. He said he was still deciding what to do next and looked to build relationships with anyone who could help. “It ain’t easy leaving football,” he said. “Because that’s my main thing, just trying to find self and put out love.”

Thomas made plans to launch a foundation to help single mothers. He had earned $75 million playing football and invested some of that in various businesses. He wanted to build a compound where his entire family could live.

But he also isolated himself and was taken advantage of by former friends.

His parents said Demaryius stopped returning their text messages and calls, and Bobby recalled that his paranoia grew to the point that he never left home without a gun.

After Thomas died on Dec. 9, family members found that cash, guns and football memorabilia had been stolen from his home. Police arrested several men who had been hangers-on during the last year of his life.

Thomas’s death shocked his former teammates, who sought ways to remember him publicly. Manning started two scholarships — one for Denver-area students, another at Thomas’s alma mater, Georgia Tech. Von Miller, playing for the Los Angeles Rams last season, wore a T-shirt with Thomas’s picture during warm-ups in the playoffs and dedicated the team’s Super Bowl win to him.

Fowler, Thomas’s former mentee, said he and many players believed they had some form of C.T.E. “It comes with the game,” he said, acknowledging that they all balance that risk with football’s life-changing benefits. Thomas was supposed to attend Fowler’s wedding this year. Instead, Fowler ended up being one of Thomas’s pallbearers.

Thomas’s parents are just now finding catharsis in talking about their son. Smith is helping Dublin city officials plan Demaryius Thomas Day on July 16, during which residents will release 88 balloons — Thomas’s Broncos uniform number. She hears about many anonymous donations her son made around town: shoes for children, turkeys on Thanksgiving.

Parents here also ask her for advice on whether to let their children play football. Thinking back to that photo of her son after Super Bowl 50 and how he reached the top of his profession only to slip away, she warns them to be careful.

“This is a once-in-a-lifetime dream come true,” she said. But “now I’m more adamant about like, hey, educate yourself on this.”