In a deal expected to be confirmed in a California courtroom, prosecutors agreed not to seek the death penalty against Mr Kaczynski, leaving him to face life in jail. The plea bargain came just as the trial was due to start before a jury in Sacramento after weeks of delays and questions about the sanity of the former mathematician who waged his bombing campaign from remote isolation in a Montana cabin.
For the US government, yesterday's deal avoided a messy and muddled case against a defendant who though found legally competent to stand trial had been diagnosed by a state psychiatrist as a paranoid schizophrenic. For Mr Kaczynski's defence - and for his brother, David, who turned him in to the FBI - it was an immense relief, and in some ways a major victory. Plea talks had been running for weeks between the two sides as the jury selection process got under way in the trial.
The sticking point, apparently, had been Mr Kaczynski's insistence on retain rights of appeal against the seizure of evidence from his cabin which pointed conclusively his to his guilt. He has now dropped that demand. In return, prosecutors will not seek the death penalty in either California, or New Jersey, where another fatal attack occurred. It remains to be seen whether state prosecutors will pursue capital cases against him.
An agreement came yesterday on the day that opening arguments were scheduled to start, soon after Judge Garland Burrell had ruled that Mr Kaczynski could not represent himself at trial, as he had requested, and it was clear that his long-delayed trial would finally go ahead.
For weeks, Mr Kaczynski has been quarrelling with his own lawyers' plans to present him as mentally ill. But Judge Burrell described his demand to act as his own lawyer as "an obvious attempt by him to purposely delay the proceedings. It is unacceptable". In Washington this week, President Bill Clinton had said Mr Kaczynski's case should be considered by a jury, because "if he's guilty, he killed a lot of people deliberately".
But the case had tested the limits of the government's resolve to seek execution of a defendant who was highly intelligent but by all measures mentally unstable. Two weeks ago, he reportedly tried to hang himself with his underwear.
Mr Kaczynski was arrested in the spring of 1996 at the door of the 10ft by 12ft hut in western Montana from where he ran a bizarre bombing campaign in a mystery that tantalised the country for nearly two decades. He was charged in California in four attacks that killed two people. His targets were picked seemingly at random from a list of university researchers and air line executives (earning the investigation its code name Unabom), and later included computer researchers and a timber company executive.
In a 35,000-word manifesto, which major US newspapers agreed to publish in return for a halt to the bombing, he justified the attacks as an attempt to spur a revolution against the encroachments of technology and the US industrial system.