At least one in four former NFL players who died between 2016 and 2021 had chronic traumatic encephalopathy (CTE), according to a new study released on Tuesday. 

CTE is a brain disease linked to repeated blows to the head, CBS News reported. CTE can only be diagnosed by examining brain tissue after someone dies. 

The study also found that nearly two-thirds of former NFL players with CTE who donated their brains for the research met the clinical criteria for dementia, and severe CTE was associated with dementia more frequently.

The study was done by Mass General Brigham, Boston University and the Concussion & CTE Foundation, and was published in The British Medical Journal.

An NFL spokesperson told CBS News that it strives to make the game safer and reduce head impacts.