“Oh, Mrs. Churchill, do come over. Someone has killed father.”

With those words — called across a yard to a neighbor, on a summer day in 1892 — a woman named Lizzie Borden entered history as villain, victim, punch line and the media sensation of the Gilded Age.

By the next morning, 1,500 gawkers had gathered outside the Borden house in Fall River, Mass. There was soon speculation that Jack the Ripper had come to America.  Someone had killed father — and stepmother, too. Their bodies were discovered hacked to death; his, lying on the couch where he had been napping; hers, face down in the spare room, bludgeoned almost twice as many times.

“There was something about the locked-room mystery of the Borden murders that turned everyone into an amateur detective,” Cara Robertson, a former SCOTUS law clerk,  writes in “The Trial of Lizzie Borden,” in her enthralling new book, almost 20 years in the making. A former legal adviser to the International Criminal Tribunal for the former Yugoslavia, she draws upon court transcripts, unpublished reports and Lizzie’s recently discovered letters to tell the story chronologically, from murder to verdict to the case’s long, strange afterlife.

Robertson does not work for the prosecution or the defense. She marshals us to no conclusion. She only reopens the case and presents the evidence afresh, all those alluring details out of an Agatha Christie novel (the mystery of Lizzie’s burned dress, the curious disappearance of a hatchet handle). The reader serves as judge and jury.

We begin in the singularly unhappy home ruled by Andrew Borden, a dour, tightfisted patriarch. His first wife died while the couple’s daughters, Emma and Lizzie, were young, and he married Abby Gray soon after. Andrew sought a mother for his children, and Abby, considered a spinster at 37, longed to leave her crowded home. It was a bad bargain for both. Little warmth existed between the couple, and Andrew gave his wife the same paltry weekly allowance as his children, never wearing a wedding ring.

Lizzie and her sister reacted to their stepmother as a usurper, and Andrew treated her little a servant. By the time the girls were in their 30s, open hostility reigned, and the family maintained parallel lives. Meals were served in two sittings; the daughters refused to dine with the parents or even talk to Abby.

These stories came tumbling out during the investigation. Robertson evokes the conditions that could birth and sustain such rancor — the narrow world of Emma and Lizzie Borden. They were too old for college or to marriage and too highborn to work. Lizzie dabbled in church activities, but “there is little evidence she found these activities satisfying.” She spoke wistfully of freedom, but she was effectively immured.

When Andrew began to suspect that one or both of his daughters had stolen valuables from him and Abby, he implemented an elaborate system of locking his room and protecting his possessions. The house began to feel like a cell. Robertson explains that Lizzie’s testimony of her activities on the day of the murder was regarded as so bizarre because the police and prosecution could not conceive of the lack of purpose in the lives of unmarried women of her class.

The case seized the town like a sickness. “Where to Look for Your Wife” ran a headline in the Fall River Daily Globe, over an item describing the “crowd of morbid females who are storming the door of the county courthouse.” Some locals called for the mystery to be solved, lest the town go insane. The murders were especially disconcerting not just because they were so savage and intimate, but because Lizzie’s arrest “unsettled an ethnically and class-determined model of criminality,” Robertson writes. In the public imagination, the white, Christian daughter of a powerful scion was considered morally incapable of such crimes.

Identity — Lizzie’s gender, in particular — became the bedrock for the cases made by both prosecution and defense. “The youngest daughter?” her lawyer petitioned. “The last one whose baby fingers have been loving entwined about her father’s head. Is there nothing in the ties of love and affection?” The prosecution, meanwhile, pointed to the nature and number of the blows, clearly committed by “an irresolute, imperfect feminine hand.”

The conspicuously unnerved judge was forced to employ a thought experiment: “Suppose for a single moment that a man was standing there,” he said. “Would there be any question in the minds of men what should be done with such a man?”

“Most interpretations tell us more about the preoccupations of its chroniclers than any essential truth about the mystery,” Robertson writes. In the 1950s, Lizzie Borden was resurrected as a feminist heroine. As one book from that period put it:

“If today a woman has come out of the kitchen, she is only following Lizzie, who came out of it with a bloody ax and helped start the rights-for-women bandwagon.” In the 1990s, another theory surfaced: that Lizzie murdered her parents after years of sexual abuse by her father. This theory relied on the fact that their bedrooms were connected by a door, the intensity of their relationship and the detail that instead of a wedding ring, Andrew wore a gold ring given to him by Lizzie.

Every generation reframes the story in the light of its signal preoccupations. Robertson does, too, and openly and she is  a scrupulous writer who stays tethered to the archives, but I often wished she had permitted herself to rove more freely, to speculate and imagine. The real riddle of Lizzie Borden isn’t whether she did it, or but the dark fascination she exerts. She is as elusive as the photographs we have — a round-cheeked young person with a decisive mouth, not quite meeting our eye.

“There seems to be little prospect that the mystery will be cleared up by the trial,” this newspaper commented prophetically in 1893. “The verdict, if there shall be a verdict, will make little difference.”