Letter bomb
Unfortunately for him, his brother recognised the writing style and alerted the police, which ultimately captured Kaczynski and he was sentenced to life imprisonment. The bombs were very well made and had no evidence from the sender, but typically of letter bombs, they are too small to case significant damage, and in most cases did not even kill the person who opened the package (this was also the case with Ekenberg, who failed to kill a single person). For this reason, and because most important government departments X-rays packages, the method is not very efficient.