Chronic traumatic encephalopathy, or CTE, is a brain disease linked to repeated hits to the head, often seen in military veterans and athletes including football players, hockey players and boxers.

It causes symptoms, which usually appear years after the head trauma, including memory loss, mood changes, confusion and trouble thinking clearly.

The 27-year-old man who allegedly shot and killed four people at a Midtown Manhattan office building on Monday carried a note in his pocket claiming he suffered from the disease, police sources told ABC News, though there is no evidence so far to confirm he had the condition.

Suspect Shane Tamura died by suicide after the Monday mass shooting at 345 Park Avenue, police said. Tamura, 27, played high school football in Los Angeles but did not play professionally, according to sources. Police have found no evidence so far that he suffered a traumatic brain injury, sources said.

There is currently no evidence of a relationship between the suspect and the military or the NFL.

CTE cannot be diagnosed in a living person with certainty, but doctors may suspect it based on symptoms and history of head trauma.

The only way to confirm CTE is through an autopsy by specialists who look for specific changes in the brain.

Researchers analyze the brain’s post-mortem to look for changes to tau proteins, found in neurons, that define CTE.

The Boston University UNITE Brain Bank holds more than 1,500 donated brains, including more than 800 confirmed cases of CTE.

Most donors were athletes or military veterans with a history of repeated head impacts.

ABC News’ Aaron Katersky contributed to this report.