With echoing chants of: “Have mercy,” three priests escorted brothers Tim and Pete Barrett to twin gallows erected in the downtown Minneapolis jail on March 22, 1889. Just before hoods were placed on the doomed brothers’ heads, one of the priests kissed 18-year-old Pete on the cheek.

“It was a signal of such touching pity as to occasion an audible moan that swept through the crowd of spectators like a shudder,” Minneapolis Journal reporter Smith B. Hall recalled 25 years later.

The Barrett brothers hanged for the murder of Thomas Tollefson, a 28-year-old Norwegian immigrant who drove the mule-powered Cedar Avenue streetcar on July 26, 1887. They fatally shot him just after midnight during a fare-box robbery that netted the brothers about $20 (worth $640 today.) A third brother at the violent scene, Henry “Reddy” Barrett, testified against his brothers and walked away scot-free.

“Oh, my God!, it is terrible to tell on my own brothers,” Henry said, “to tell what will hang them, and perhaps me, too.”

But roll over he did, eluding prosecution thanks to a deal struck with Hennepin County Attorney Robert Jamison.

The hangings sparked a push to reform capital punishment in Minnesota, which the Legislature banned 12 years later.

The Barretts’ double hanging ended what the St. Paul Daily Globe called the “most thrilling, dramatic, diabolical and utterly incomprehensible murder case known to the criminal annals of Minnesota.”

Now more than 135 years later, a retired Golden Valley financial executive named Gary Heyn has stumbled upon a troubling wrinkle in the case. Heyn, 67, recently published a nonfiction novel titled “Standing at the Grave,” about his family’s history from Prussia to Minnesota to North Dakota (www.tinyurl.com/HeynBook).

During his research, Heyn discovered that his third great-uncle, Julius Heyn, was the only eyewitness to Tollefson’s slaying. But he testified for the defense only at Pete’s second trial — offering a different account ignored during the first trial.

“At first, I thought he was just a minor figure in the trial,” Heyn said. “But the more I dug in, the more I realized he played a key role in the case.”

A German immigrant in his early 30s who sold insurance, Julius Heyn testified in Pete Barrett’s case that he returned to his home at 3009 Cedar about midnight, put on his slippers, went into his yard, heard a scream and saw a pistol flash after one shot. He ran toward the commotion and heard “three more shots in rapid succession.”

Julius Heyn insisted the four shots came from the same gun — a marked difference from Henry’s story of two shots, one each from his brothers’ guns.

“Mr. Heyn’s testimony … makes it more probable that Peter will be freed,” the Minneapolis Tribune predicted, saying Heyn “proved a good witness” whom jurors might believe more than Henry — who was testifying to avoid getting prosecuted himself.

A legal spat erupted over why Heyn hadn’t testified at Tim Barrett’s earlier trial when he gave the same account during a coroner’s inquest. Maybe the prosecutors didn’t want to muddy Henry’s confession?

“In the end, the jury believed the brother who’d made a deal over Julius, the only eyewitness,” Gary Heyn said.

The Barrett brothers came to Minneapolis from Omaha about 10 months before Tollefson’s murder and “all of them were choice young desperadoes … and their characters were steeped in the juices of iniquity,” according to the Minneapolis Tribune. Tim Barrett had been arrested for highway robbery and Henry was jailed for running an unlicensed saloon.

Three months after the unsolved slaying, their mother ratted them out — angry that her sons stole her pony and sold it in Iowa. Henry then rolled over on his brothers.

Henry Barrett testified that he was unarmed, except for a pool cue, while his brothers carried handguns when they headed out on July 26, 1887.

At 26th and Cedar, the brothers tossed some planks on the streetcar path. When they encountered Tollefson at the end of his shift, they demanded his cash box. After Tollefson struggled, Henry Barrett said his brother Pete shot him in the leg. They ran off to a cemetery nearby, soon joined by Tim. Henry said that Tim admitted, “I killed him. … I shot him through the head.”

That was enough to convict Tim in the first trial, which lasted 16 days in a courtroom “crowded to suffocation.” Pete, who was only 16 the night of the crime, was convicted in a 21-day trial in 1888.

Despite differing accounts and a witness omitted from the first trial, Judge William Lochren declined to order a new trial, as defense lawyers requested. Gov. William Rush Merriam refused to issue a reprieve.

So on the first full day of spring 1889, Hennepin County Sheriff James Ege “adjusted the knots just beneath the left ear of each,” according to a reporter covering the execution. “There was a creak and a bang of falling traps and a drop of two human bodies.”

Curt Brown’s tales about Minnesota’s history appear every other Sunday. Readers can send him ideas and suggestions at mnhistory@startribune.com. His latest book looks at 1918 Minnesota, when flu, war and fires converged: strib.mn/MN1918.