The disease has been diagnosed in more than 100 former NFL players and arisen as an existential threat to the United States’ most powerful pro sports league.

Tamura killed four and badly wounded another before shooting himself in the chest.In the note, Tamura claimed to have CTE—chronic traumatic encephalopathy, a brain injury condition due to repetitive trauma to the brain—and that he wanted researchers to study his brain.

The man who killed at least four people in a N.Y.C. office building shooting claimed in his suicide note to have chronic traumatic encephalopathy (CTE). Here’s what to know about the degenerative brain disease.