According to the Mayo Clinic, CTE — or chronic traumatic encephalopathy — is a brain disease most likely caused by repeated head injuries that damage the brain's nerve cells. The Mayo Clinic notes the only way to tell for sure if someone suffered from CTE is by autopsy.

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.

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.