CLARKSVILLE, Tenn. — Months later, the father mustered the strength to sort through what was left in his dead son’s bedroom. A Little League photo collage. Mardi Gras beads from that soccer tournament in New Orleans. And a typewritten personal essay tucked into a yellow folder, with a single word pen-carved into its plastic cover:

“Concussions.”

His son Curtis had written the paper for a college composition class in 2009, five years before his death. In it, Curtis recalled having been knocked unconscious three times in 14 years while playing soccer, twice after leaping to head the ball, only to — “WHAM” — collide with another player. The continuing side effects, he wrote, included “horrendous migraine headaches.”

“If I feel this way now,” he wondered, “what will it be like when I’m older?” He was 19.

Reading those words, his father, Bill Baushke, felt the floor drop beneath him. This meant that Curtis knew. He knew well before most of us that repeated concussions could also lead soccer players — and not just those who box or play football — down the dark spiral of cognitive damage and decline.

Well before the death in 2012 of the semiprofessional soccer player Patrick Grange, 29, who was posthumously found to have chronic traumatic encephalopathy, the degenerative brain disease linked to concussions. Before the 2014 death of Hilderaldo Bellini, 83, the retired Brazilian soccer star, also found to have C.T.E. Before the specter of C.T.E. began to loom over every contact sport.

Curtis Baushke, racked by migraines and struggling with focus, was still focused enough to know. “Allowing kids to play too soon after a concussion could be very dangerous,” he wrote. “We need to find out the actual damage concussions cause people.”