Gilbert Murray, president of the California Forestry Association, died when he opened a heavy package about the size of a shoe box delivered to the association's office in Sacramento on Monday.
As always with "Unabomber", the blast came without prior warning. Similarities with previous unexplained bombings, 15 since 1978 across the US, "strongly suggest" the Sacramento killing is the work of the same person, FBI special agent Richard Ross said, emphasising that there was no sign of any connection with the Oklahoma City attack.
Despite 17 years of pursuit, Bureau specialists have come up with only the haziest Identikit of their target, usually depicted as a white man in a ski hat and dark glasses. He is believed to be about 40, well educated, and clearly an expert in his field.
Mr Gilbert's death, coming four months after the murder of a New Jersey employee of an advertising agency working on pro-smoking issues, could mean the postal bomber is a pro-health and pro-environment fanatic.