HANOVER -- The crime scene was grisly.

A woman’s body lay face-down outside the old rambling farmhouse in Halifax. When police arrived, they opened the back door and found a man – Thomas Sturtevant — his head smashed in. Dead.

In a first-floor bedroom, Sturtevant’s younger brother Simeon had suffered the same fate, his blood splattered against the walls. A triple homicide.

Police immediately suspected the wealthy brothers’ grandnephew, William Sturtevant, a shady character and a suspected thief who worked at a nearby shoe factory in Whitman.

When they confronted him, they noticed blood on his collar and on his hat. And on his coat. The guy had a million excuses, but it was the beginning of an evidence trail that pointed toward guilt.

“It’s detective work for me,’’ John F. Gallagher, who wore the badge of the Boston Police Department for 29 years, told me the other day.

“It’s digging up these facts and discovering these things that nobody ever knew at the time. I’m basically putting together a package for the district attorney to prosecute the case in front of a grand jury.’’

Except the cases, like the 1874 Halifax murders, are more than a century old.

Gallagher’s detective work, which he used to conduct on the streets of Boston, now takes place in university libraries, state archives, and in front of microfilm machines where he pieces together parts of a puzzle that — through two books now and a third about to be published that examines the Halifax killings — add up to cold-blooded murder.

“It’s dark history,’’ he told me across his kitchen table here. “You’re not going to make a lot of money doing it. But it’s kind of like a community service. And it’s a way to honor the memory of these murder victims long ago.’’

It’s also sort of a bookend to a career in police work that had its own bloody beginning.

Gallagher, the oldest of nine children, was born in South Boston. His father was a traffic manager for a trucking firm. His mother was a telephone operator. And the family was friendly with another man named Gallagher, John J. Gallagher.

That Gallagher, a Boston police officer, former Marine, and father of three young children, was just 33 in 1962 when he was gunned down by a bank robber after he responded to an alarm at what was then the National Shawmut Bank in Kenmore Square.

“He had made a huge impression on me,’’ Gallagher said of his namesake. “I’m sure that’s why I became a policeman.’’ And in June 1979, that’s what he did.

John F. Gallagher worked his way up the ranks of the BPD, making sergeant in 1988. Later, as a lieutenant detective, he supervised the drug unit, which he was asked to “tighten up’’ after a police SWAT team in search of drugs mistakenly broke down the door of the Rev. Accelynne Williams in 1994. They broke in without warning and handcuffed the 75-year-old minister who, minutes later, died of heart failure.

Gallagher later was assigned to the major case unit, investigating organized crime. He was promoted to superintendent in 2000, the same year he donated part of his liver to his mother, extending her life by a year – enough for her and his father to celebrate their 50th wedding anniversary.

“How many of us have an opportunity to save the life of a parent, one who has given us life?’’ Gallagher asked. “If I could risk my life for strangers, how could I not risk it for my own mother?’’

After his 2008 retirement, he took deep dives into history, writing books chronicling the 1913 poisoning death of a retired Navy admiral, whose wife stood trial in Plymouth before an all-male jury, and the 1845 shooting deaths of two Irish railroad workers by an illegal rum dealer in Hanover.

“I found a lot of excellent police work,’’ he said. “What was different is that people then were more willing to come forward to testify about what they had seen. Today, people are more reluctant to come forward for whatever reason. But in those days, it seemed like they got a lot more cooperation from witnesses.’’

Gallagher turned 65 last month. When he’s not watching his five grandkids, he can be found writing and researching in his basement office.

On the wall are framed mementoes of his police career. All the badges that had been pinned to his chest – from patrolman to superintendent – are there.

It’s where he’s still on the job, combing through musty archives, tracing genealogical histories, reviewing trial transcripts – the long arm of the law reaching back 100 years in search of the truth.

Thomas Farragher is a Globe columnist. He can reached at thomas.farragher@globe.com.