and dropped the plane out of the sky. >> reporter: authorities now begin to suspect the bombings are linked. all doubt is removed seven months later. a suspicious package arrives at the home of percy wood, the president of united airlines. the subsequent explosion inflicts cuts and burns over large portions of wood's body. >> now, come 1980, we know we have a serial bomber. and so, the fbi started working as a joint task force with atf. and because we had bombs in the mail, with the postal inspection service. >> reporter: the task force dubs the investigation una bomb. >> universities and airline bombings because those first four bombs were either affiliated with university locations or with airlines. >> reporter: from the beginning, the investigation is hampered by a lack of evidence. the unabomber's devices are relatively crude, making it difficult to trace them back to