A degenerative brain disease that has been linked to concussions and other head trauma common in military combat and contact sports, CTE 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.

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.

No ruling has been made yet on whether Tamura was suffering from the disease chronic traumatic encephalopathy (CTE), which he claimed to have had.