An Iowa letter carrier was sentenced to 37 years in prison Tuesday for sending dud pipebombs with letters signed "The Bishop" in an odd but potentially deadly bid to drive up the value of shares he owned.

John Tomkins, 48, showed little emotion as a federal judge in Chicago imposed the sentence. Later, before marshals led the Dubuque, Iowa, man away in handcuffs, he smiled as his attorney patted his shoulder.

In an hourlong preamble to the sentence, Judge Robert Dow praised Tomkins for taking some responsibility but added the father of three "engaged in a reign of terror" in his mailings to investment firms and advisers.

"'Horrific' is the single best word I've heard to describe this crime," Dow said. "'Terrifying' is another good word."

Tomkins got the idea to sign his letters "The Bishop" from a novel in which a criminal leaves a chess piece as his calling card. His notes read, "BANG! YOU'RE DEAD," and said the only reason the recipient wasn't dead was because a lone wire wasn't attached.

Tomkins faced a mandatory minimum sentence of 30 years, though prosecutors asked for around 45 years. Counting six years Tomkins has already served and credit for good behavior, he could be released by his mid-70s.

The acting U.S. attorney in Chicago welcomed the sentence.

"Tomkins took these terrifying and secretive actions because he was greedy," said Gary Shapiro. "He was indifferent to whether he killed people in the process."

Jurors convicted Tomkins last year on 12 counts, including the use of a destructive device while mailing threatening communications. Combining all the maximums, Tomkins faced a sentence of more than 200 years.

Tomkins' lawyer, Francis Lipuma, told reporters after sentencing that he plans to appeal the convictions and portions of the sentencing. But he conceded the sentence could have been far worse for his client.

"He's a family man and a man who was respected in his community," Lipuma said, adding the judge recognized that in not imposing a harsher sentence.

Tomkins' wife, Julie, was in court but declined comment to reporters later.

Dow said he was perplexed about what led Tomkins to do what he did, saying he seemed to live a typical, small-town American life not unlike the community Dow said he grew up in. He even cited Tomkins' fondness for bowling, garnering a smile from Tomkins.

"The defendant's secret life" planning his crimes from storage garages and his car, Dow said, "comes seemingly from nowhere."

Tomkins did not address the court Wednesday, which was scheduled only for Dow to announce the sentence. But during a first phase of sentencing last month, Tomkins apologized for what he'd done.

"Let me start by saying how incredibly sorry I am," he told Dow. "There are no words to describe the shame and disappointment I feel in myself."

Authorities spent two years trying to track down "The Bishop," eventually identifying him as Tomkins in 2007 using stock market records on the two firms he cited in his letters — 3COM Corp. and Navarre Corp.

To make the letters harder to trace, Tomkins drove from Iowa to mail two packages from the Chicago area in 2007. In a dozen letters, Tomkins threatened to kill recipients, their families or neighbors unless they acted to raise the stock prices.

The former machinist represented himself at trial, portraying himself as a mild-mannered union man fond of building race cars. He's also blamed the suicide of his nephew and the killing of a friend for triggering "a mental breakdown."

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Tomkins also insisted at trial that he carefully designed the ominous-looking devices so they could never explode, but prosecutors said the pipe bombs were close enough to fully operational explosives and that it was just "dumb luck" they didn't go off.

Serving as his own attorney led to the strange spectacle of Tomkins calling himself to the stand and referring to himself in the third person. In his closing, he apologized for his lack of legal training and asked jurors to "not hold my shortcomings against the defendant when it comes to being a lawyer."