In 2002, UK soccer fans—or rather football fans—were shocked when a coroner determined that celebrated retired player Jeff Astle, 59, had died of dementia caused by a long career heading the soccer ball. Heading is, of course, central to playing soccer: An average player heads the ball between six and 12 times per game, adding up to at least 2,000 headers during a 20-year career, not even counting heading drills during practice.

But despite Astle’s case, connecting soccer heading to long-term and even deadly brain damage has yet to get the American football treatment, which has been the subject of a torrent of news stories and legal cases connecting American football-induced head trauma to players’ chronic traumatic encephalopathy (CTE), dementia, and Alzheimer’s.

Now that may be changing: In a paper published Tuesday in the journal Acta Neuropathologica, University College of London researchers who conducted post-mortem exams of the brains of six former professional UK soccer players found all six had Alzheimer’s. Four had diagnosable CTE, a condition characterized by repetitive mild traumatic brain injury that can be seen from a buildup of tau protein in a distinct pattern on the brain. In life, CTE causes behavioral changes, memory loss, impaired motor skills and slurred speech, sometimes decades after the period when trauma occurred. A diagnosis can’t be confirmed until after death, though, when scientists can look directly at the brain.

“This is the first time CTE has been confirmed in a group of retired footballers,” lead author Helen Ling, of the UCL Institute of Neurology, said in a statement.

In total, the team of researchers followed 14 retired soccer players from 1980 to 2010, and six of the players’ families allowed the researchers to examine their brains after they died. All 14 had dementia (which began on average, around their 64th birthday), and had played soccer professionally or at a high level for an average of 26 years. All began playing as children or teens. Six reported ever having a concussion with loss of consciousness in their careers, and each reported only one concussion of that kind.

Twelve out of the 14 eventually died of advanced dementia.

Neuroscientists have already expressed concern that young children who play soccer are at risk of brain injury from heading the ball, and a 2016 study found instant changes to brain function after a single bout of heading practice, where adult players in a clinical setting headed the ball 20 times and then took cognitive and memory tests. They scored lower on both tests than they had before practice, and brain function normalized after about 24 hours. But the long term effects, the paper’s authors noted, were still an open question.

While the study results certainly suggest a link between soccer and late-life degenerative brain diseases, the sample size is too small to determine conclusively that the former causes the latter. But even in the case of American football—where there have been studies with much larger sample sizes, like one lab that showed 90 former NFL players had CTE—the science is still unsettled. Researchers still don’t know how much risk each player has of developing CTE, or why some players who experience head trauma don’t get it, according to the New York Times. More study is needed—and the same is true of the soccer research.

“[O]ur findings support the need for further systematic investigation, including large-scale case–control studies,” the University College of London researchers wrote, noting that the the potential connection would be “of considerable public health interest.” After all, roughly 265 million people of all ages play soccer globally, according to Fifa’s last major survey, conducted in 2006.