The FBI captured the Chicago-born in 1996, and he was serving life in prison when he died. The Department of Justice issued a statement, saying he died at the federal prison medical center in Butner, North Carolina. A source told ABC News that his death is being investigated as a suicide.
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Kaczynski started his bombing spree in Chicago before waging a campaign of terror that set universities and airlines on edge nationwide. According to the FBI, he admitted to committing 16 bombings from 1978 to 1995, killing three people and permanently maiming several of his victims.
Thomas Mockaitis is a DePaul University Historian and terrorism expert. He said there are lessons to learn from a lone wolf terrorist like Kaczynski.
"He got his title before they figured out who he was as the university and airline bomber, hence 'Unabomber,'' Mockaitis said. "It took them about 18 years to catch him, and that was only because his brother recognized the writing style of a manifesto that was partially aired."
Kaczynski was the son of a sausage maker and a homemaker. His first bomb was left in May of 1978 on the University of Illinois - Chicago Circle Campus before it was returned by a passerby to a Northwestern University professor. The package exploded upon opening and injured a security officer.
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"I think there was a huge sigh of relief when he was finally caught to realize that he was truly the old-fashioned lone wolf. No known connection to anybody. No known connection to any broad ideological movement. So, no reason to fear that there would be a repeat of these incidents," Mockaitis said.
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But, a cautionary tale for law enforcement continues.
"It's still very, very hard to catch a dedicated individual and who is determined to engage in violent behavior, and especially if they don't mind being caught," Mockaitis said.
Before Kaczynski's transfer to the prison medical facility a year and a half ago, he was held in the federal Supermax prison in Florence, Colorado.