An Iowa letter carrier has been sentenced to 37 years in prison for sending dud pipe bombs with threatening letters signed “The Bishop.”

U.S. District Judge Robert Dow issued the sentence Tuesday afternoon, saying John Tomkins “engaged in a reign of terror.”

The 48-year-old Dubuque, Iowa man previously apologized, saying there were “no words to describe” his “shame.”

Tomkins was trying to drive up the value of shares he owned. One package was sent to Denver and another to Kansas City, Mo.

Jurors convicted Tomkins last year. The most serious charge was for use of a destructive device while mailing threatening communications. It carried a mandatory sentence of at least 30 years.

Tomkins last month said the mandatory minimum was unfair. Prosecutors sought a sentence of around 45 years.