In the 1920s, forensics was in its infancy, and investigators often found themselves pitted against the policeblogs at Speakeasy Science

A winter mist settled in that first week of December 1926. So the police officer patrolling the Brooklyn wharfs almost missed it, like a shadow sliding by, a man edging toward the river. The shadow man seemed oddly misshapen, bent forward by a bundle braced on his back.

At the edge of a pier, the bundle dropped with a wet thud. Curious, the officer shouted an order to wait there. Instead, the man kicked his bundle into the dark water. Stop, the patrolman shouted, stop: but now the shadow man was running, running until he was tackled on to the wet wood of the wharf.

In the light of the precinct station, the police pelted him with questions. They'd identified him as a dockworker, Francesco Travia, middle-aged, scruffy and silent. He just sat in his chair, staring at his shoes. His sodden shoes and his socks, red with blood. They locked him away and went to his apartment, wondering, as you might expect, about that blood.

It was worse than they'd expected. A dead woman sprawled on the kitchen floor. Or rather half a dead woman. The upper part of body – torso, arms, head – in a clotted pool of red. A spattered butcher's knife and a chisel lay on a table smeared and streaked with gore.

The officers hurtled back to the station and arrested Travia on murder and dismemberment charges. They would turn out to be only half right in their conclusion. The Travia case – in which scientists and policemen took opposite sides – is mostly forgotten today. I only stumbled into it while researching my latest book, The Poisoner's Handbook in which, frankly, I took a kind of pleasure in exploring lost tales from forensic history.

And obscure as it is, it remains one of my favourite examples of the way criminal science evolved and, as it did so, changed the way we live in today. In our modern CSI world, we take for granted that scientists and police detectives work side by side toward a mutual goal. But early in the last century, not that long ago, such harmony didn't exist. Police detectives and chemical detectives were mostly still strangers, working in different worlds, especially in North America. But, incrementally, Travia case by Travia case, chemical by experiment by experiment, a partnership formed.

And changed everything.

The New York City medical examiner, Dr Charles Norris himself, was on call the night of the Travia arrest. He followed the policemen up the wooden stairs to Travia's apartment, walked over to inspect the dismembered corpse.

His thick eyebrows drew together. The blood pooled around the half-body was a bright cherry-red. He bent to look closer at the woman's face. It was flushed pink, despite the massive blood loss. As told by a crime writer, Norris walked over to the waiting detectives and announced: "Boys, you can't hold this man for murder."

The Brooklyn police assured him that they could – and would.

The mutilated body went to the city morgue where it was identified as a Brooklyn woman, Anna Fredericksen, who ran a rooming house around the corner from Travia's apartment. Her family was horrified. They knew Frank Travia; considered him a friend. Travia was a loner, sure, a drinker, yes, but Anna was a drinker too. No one thought of him as a violent man. And their Anna was plain easygoing, not the kind of woman to provoke a murder.

Norris's reaction to the corpse came from a simple fact: people killed by the poisonous gas carbon monoxide tend to flush pink, the result of a chemical reaction in the blood. A murder victim who bleeds to death would have been porcelain pale. He was almost positive that she'd been poisoned before being cut apart.

So he asked his forensic chemist – a dark, slight, completely obsessive toxicologist named Alexander Gettler to run a series of tests. Back in the mid-1920s, one of the simplest ways to test for carbon monoxide was to extract blood, pour some of it into a porcelain dish, and stir in some lye. Lye turned normal blood into a dark, gelatinous ooze which, when held to the light, showed murky layers of greenish-brown. But blood saturated with carbon monoxide stayed a chemically altered crimson.

Chemists weren't sure exactly what produced that contrast but they suspected it had something to do with the relentless grip that carbon monoxide exerted on iron-rich proteins in the blood. And, relentless was the word. Left for weeks during time-tests, residing in stoppered bottles on the wooden counters of Gettler's lab, blood solutions tainted by carbon monoxide glowed like the crimson hourglasses on a black widow spider, like warning lights signalling danger to those who got too close.

Gettler drew blood directly from Anna Fredericksen's heart. The solutions flamed engine red. And as corpses don't absorb the gas, and as the saturation level was lethal, their dismembered corpse had been dead, Gettler reported, before Travia had picked up his knife.

As Travia told investigators – in a sudden, frightened confession – Anna Frederickson had come by searching for whisky. Her husband admitted to police that she "frequently drank intoxicants" and that her usual bootlegger had been unable to deliver that weekend.

Travia and Anna finished his supply of liquor, sitting at his kitchen table and he thought they'd fallen into an argument when he tried to get her to leave, and then, well, he'd felt incredibly sleepy and had fallen asleep at the table. He woke, sometime later, he wasn't sure, foggy-headed. She was still there, lying on the floor. He went to shake her awake. She was creepily cold to the touch, creepily stiff.

He was slow, dizzy, his head filled with that winter mist. He could only think, he explained, that he must have killed her while they'd argued, shaken her to death, strangled her, he didn't know, but he did know there was a dead woman on the floor. He did know that he'd be charged with murder once she was found.

So there in the dark of early morning, he made a decision that he could stay out of trouble by getting rid of the body. She was a big woman, tall and chunky, too large to simply haul away over a shoulder. Desperate, Travia decided to cut her into halves, using a butcher knife and chisel, and then get rid of her, one part at a time.

He wrapped the lower half of her body in newspapers, burlap bags and an old raincoat and carried it down to the river. He hadn't figured out what to do with the upper half but then he never got that chance. That was all, he swore. And whether or not one believed him depended on how much one believed in the scientific results from Bellevue.

And Norris could talk all he wanted about the significance of pink faces; Gettler could discuss lethal saturations until he grew hoarse. But in Brooklyn, the police found bloody knives and body parts more believable. A lot more.

Which meant that Francesco Travis was arraigned to stand trial on murder. And – at least as Charles Norris saw it – that the New York City Medical Examiner's Office was also on trial, needed to prove that scientific evidence was a tangible thing, as real, as convincing – and as influential – as any other evidence presented in a courtroom.

Unlike the police, Travia's lawyer found the medical evidence compelling. In fact, the defence called only three witnesses: the building owner to say that he'd discovered that a coffee pot on Travia's stove had boiled over, putting out the burner flame, allowing gas to drift through the apartment; Alexander Gettler to testify that carbon monoxide poisoning had caused the woman's death; and Frank Travia to confess his half-crazed panic on finding his neighbour dead.

In March 1927, he was acquitted of murder, convicted instead of illegally dismembering a dead body. A life-saving difference: in 1920s New York, it meant that he went to prison instead of Sing Sing prison's infamous electric chair. They celebrated at the city medical examiner, believing that the case had given them new credibility, that they'd also proved that forensic toxicology was a credible, believable tool.

And they were right. But there's another lesson inherent in its story. The goal of forensic science is more than establishing criminal guilt. Forensics done right – meticulous, honest, impartial – should also be valued because it proves innocence. And sometimes even a blood-stained Francesco Travia is not quite as guilty as he looks.

Deborah Blum is a Pulitzer-prize winning science writer and a professor of journalism at the University of Wisconsin. She blogs at Speakeasy Science