A Stoughton man was sentenced to three years in federal prison for his role in two robberies of postal carriers in Mattapan and in Hyde Park in late 2022 — with one of the robberies done at knifepoint.

Kenneth Demosthene, 24, pleaded guilty to two counts of robbery in July 2024. On Tuesday, U.S. District Court Chief Judge F. Dennis Saylor IV sentenced him to three years in prison to be followed by three years of supervised release. The sentence was quite a bit shorter than the four years and three months prosecutor Luke Goldworm asked for in his sentencing memo.

“Can I ask you a question,” Demosthene said as he walked up to a mail carrier on Hiawatha Road in Mattapan at 2 in the afternoon on Nov. 29, 2022. “I’m going to need your master key.”

Demosthenes then reached out, according to court documents, and grabbed at the carrier’s Arrow key, which was secured to the Postal Service employee with a brass chain. The carrier told investigators that Demosthenes “yanked pretty hard” on the key, enough to pop the chain.

These “Arrow keys” are used by the U.S. Postal Service to open the blue collection boxes one sees along sidewalks. Not only do people send letters by placing them in these boxes, but they also use them to mail checks — and “as a result, criminals routinely target USPS first-class mail placed in blue collection boxes to steal checks,” U.S. Postal Inspector Daniel Lee wrote in an affidavit supporting the charges.

Demosthenes then fled the area and, according to area surveillance video, entered a Zipcar to make his getaway with the master key along with his alleged co-conspirator Myesha Lewis.

Surveillance footage also saw that in addition to his black face mask, he wore a black hooded sweatshirt with the words “Lonely Hearts Club” written prominently on the back — a review of records from the producer of that shirt showed that he was only five people in Massachusetts to order the item.

The second robbery took place in Hyde Park on Dec. 16, 2022, a little before 5 p.m. and targeted yet another master key. This time, though, the robber who both victims described as pretty short and skinny didn’t leave anything to chance. He worked with a partner and switched out a roughly 3-inch blade as he approached the mail carrier.

“Give me your (expletive) Arrow key,” he yelled at the mail carrier.

Demosthenes’ actions were not unusual, but just two more in a burgeoning criminal field, according to documents filed in the case. There had been 13 assaults on letter carriers in the immediate Boston area when the charges were filed in May 2023, and according to the U.S. Attorney’s office that number had grown to 23 by the time of Demosthenes’ sentencing.

The U.S. Postal Inspection Service noting an uptick between 2016 and 2021 in “mailbox fishing” in Massachusetts. This is using everyday objects to try to “fish” letters, and hopefully checks, from the boxes if the criminal doesn’t have an arrow key.

In response, the U.S.P.S. worked to harden the boxes against “fishing-related mail theft,” but that only led to a spike in Arrow key robbery.

A Herald request for comment from the National Association of Letter Carriers, the union representing mail carriers, was not returned by deadline Wednesday.