The gunman who opened fire inside a New York City skyscraper that is home to the NFL‘s headquarters on Monday reportedly left a suicide note saying he suffered from Chronic traumatic encephalopathy (CTE) and wanted his brain to be studied.

Authorities named 27-year-old Shane Devon Tamura as the chief suspect in the attack that killed four people, including police officer Didarul Islam, and wounded a fifth. The gunman died by suicide, officials said.

A suicide note was discovered in the suspect’s back pocket that said he suffered from CTE, CNN reported, citing an unnamed source with knowledge of the investigation.

The degenerative brain disease has been linked to repeated trauma to the head, including concussions, common in contact sports like football. Symptoms can be used to diagnose suspected CTE in the living, but can only be confirmed through a post-mortem examination of the brain because it requires examination of brain tissue.

Football “gave me CTE and it caused me to drink a gallon of antifreeze,” the note read, according to CNN. “You can’t go against the NFL, they’ll squash you.”

Tamura was a competitive football player in his youth, CNN reported.

New York Police officers gather at the scene on 52nd Street outside a Manhattan office building where a gunman killed four people, including police officer Didarul Islam, and wounded a fifth on Monday, July 28. New York Police officers gather at the scene on 52nd Street outside a Manhattan office building where a gunman killed four people, including police officer Didarul Islam, and wounded a fifth on Monday, July 28. Angelina Katsanis/AP Photo

Police Commissioner Jessica Tisch said Tamura had a “documented mental health history” and that his motive remains under investigation.

If you or someone you know is considering suicide, contact the 988 Suicide & Crisis Lifeline by dialing 988, text “988” to the Crisis Text Line at 741741 or go to 988lifeline.org.

This is a developing story. Updates to follow.