Theodore J. Kaczynski, the so-called Unabomber, who attacked academics, businessmen and random civilians with homemade bombs from 1978 to 1995, killing three people and injuring 23 with the stated goal of bringing about the collapse of the modern social order — a violent spree that ended after what was often described as the longest and most costly manhunt in American history — died on Saturday. He was 81.

A spokesman for the Federal Bureau of Prisons said Mr. Kaczynski was found unresponsive in his cell early Saturday morning. The cause of death was not immediately known.

Mr. Kaczynski traced a path that was singular in American life: lonely boy genius to Harvard-trained star of pure mathematics to rural recluse to notorious murderer to imprisoned extremist.

In the public eye, he fused a rare mix of styles of violence: the periodic targeting of the demented serial killer and the ideological fanaticism of the terrorist.

A complete obituary will appear shortly.