The letters appear to have been posted from Oakland, California, at much the same time as the explosive device which killed a Sacramento timber industry executive on Monday - the third death in 16 attacks since 1978 by the individual the FBI know simply as "Unabomber": 22 other people have been injured by bombs attributed to him.
Although the letters purport to be the work of an "anarchist" belonging to the "terrorist group FC", and refer throughout as "we", they have given the clearest clue yet to his motives.
The pattern of the bombings has suggested a fanatical environmentalist or health activist, perhaps with a grudge against universities. But the rambling 2,000-word letter, published by the Times, said "the people we are out to get" are scientists and engineers, especially in computers and genetics. The author has "nothing against universities or scholars as such", but acknowledges FC's goal is to "eliminate industrial society".
"Unabomber" promises the attacks will stop for good if a far longer tract, between 29,000 and 37,000 words, is published by the New York Times, Time, Newsweek, or another nation- ally distributed periodical. The letter also contained a nine- digit number used in a previous communication in 1993.
Investigators believe "Unabomber" has only surfaced now because of resentment at the attention attracted by Oklahoma City. Police say the bomber has a bizarre fondness for wood, both to make his bombs and in his choice of victim - hence this week's mailbomb which killed Gilbert Murray of the California Forestry Association.