The 81-year-old was moved to the facility in North Carolina in 2021 because of health issues. Prison officials said only that he was found unresponsive on Saturday morning.
Kaczynski was serving four life sentences after pleading guilty to 16 bombings between 1978 and 1995 in a twisted one-man rebellion against modern life waged from a Montana shack.
Three people were killed and two dozen more were hurt during his reign of terror.
He was turned in by his brother, David, who recognized his writing style in a manifesto published by The Washington Post and The New York Times.
“It was a nightmare,” David Kaczynski said in 2005, according to the Associated Press. “I was literally thinking, ‘My brother’s a serial killer, the most wanted man in America.’”
If you or a loved one are struggling with suicidal thoughts, please reach out to the National Suicide Prevention Lifeline at 1-800-273-TALK (8255), or contact the Crisis Text Line by texting TALK to 741741. You can also text or dial 988.