On a chilly, moonless Parisian night, Jacquette Griffard was putting her baby to bed when she heard shouts in the street: “Kill him!”

This shoemaker’s wife looked down from her upper-story window and saw a gang of masked thugs slicing up a kneeling man with swords and axes. Some of the assailants held torches, lighting the horrific scene.

“Murder!” Jacquette screamed.

One of the killers looked up and yelled: “Shut up, you damned woman!”

What Jacquette had just witnessed—and soon would describe to investigators—was the assassination of the king’s brother, Louis of Orleans. Louis had periodically ruled France during the king’s frequent spells of insanity. His bloody demise plunged France into civil war and led to Henry V’s devastating English invasion that put the “Hundred” in The Hundred Years’ War.

And yet the story of how Paris’ chief of police, a precursor to Raymond Chandler’s honorable detective hero who goes down the “mean streets” but “who is neither tarnished, nor afraid,” was lost for centuries.

I first learned about the courageous sleuth, Sir Guillaume de Tignonville, a decade ago, when my wife and I rented an apartment for a month on the Rue Vieille du Temple while I worked in the Paris archives. I’m a professor of medieval literature at UCLA, and I was researching a trial by combat that took place in Paris in 1386. One day, reading up on “our” street, I learned that it had been the scene of a bloody assassination in 1407 that changed the course of history. The story grabbed me, and I began thinking about my next book.

As a murder in the royal family, this case was different from most of the crimes that Guillaume dealt with in the crowded, smelly, dangerous place that was medieval Paris. Europe’s largest city, it had at least 100,000 inhabitants, many of them beggars, pimps, prostitutes, petty thieves, and grifters roaming the city in search of clients or victims. Some of them bribed the police to look the other way, and one obliging officer even took fiddlers on his nightly rounds to play music and alert any crooks in the area.

Suspects from this Parisian underworld were often hauled into court at police headquarters, a gloomy old fortress fronting the river Seine and known as the Châtelet— “the little castle.” It had a morgue, several prisons, and an evil reputation because of the torture regularly used there to extract confessions. Methods included an early form of waterboarding combined with a sort of rack.

High-profile cases could be difficult to crack. Powerful figures always have enemies, and Paris at night was a dark labyrinth where attackers could strike by surprise and just disappear. In 1392, for example, the king’s top military officer, Olivier de Clisson, was nearly assassinated by a gang of sword-wielding thugs who were never caught. Unlike the fictional sleuths in shows like CSI and True Detective, who pack heat and have back-up at the lab, a man of the law in 1400s Paris had to rely on his wits and his encyclopedic knowledge of the city. And he had to keep his sword handy: In 1372, a law officer had been assassinated while investigating a crime.

Guillaume, who had been police chief for six years at the time of Louis’ murder, mobilized his officers at once. He ordered all the city gates closed to prevent the assassins from escaping and posted guards in the streets to keep the peace. He ordered a search of a nearby house, evidently used by the killers as a hideout. And the day after the murder, he began summoning several dozen potential witnesses. These clerks, barbers, and housewives were deposed in small rooms by two-man teams. One asked questions while the other recorded their testimony, his goose quill scratching on parchment. Details came from Jacquette, who was drying her baby’s linen by the window, a salt seller named Gilet watching from his doorway, and a young clerk who found Louis’ severed hand just outside his door.

After a number of false leads and dead ends, as well as reluctant or misleading witnesses, Guillaume found the broker who had rented the house to the assassins and several vendors who had sold them goods or supplies. He began to suspect a far-flung conspiracy behind the murder—and a deadly political rot reaching as high as the royal family itself.

Guillaume suspected Louis’ cousin, John of Burgundy, a lord who had conspicuously appeared at Louis’s funeral draped in black and boldly proclaimed, “Never was there a more treacherous murder!” At great personal risk, the police chief set a cunning trap for Burgundy: At a meeting of the royal council, he dared the lords of France to allow his officers to search their palaces, normally off-limits to law enforcement officials. All but he agreed, because they were innocent of the crime, thus trapping Burgundy into a search and prompting a sudden confession. “I did it!” he burst out right there, in front of his relatives. It was the culmination of diligent inquiry that amazingly did not appear to have used judicial torture, force, or threats. Instead, Guillaume relied on stealth, surprise, and cleverness.

We know a lot about Guillaume’s methods because of the detailed report he left behind in a 30-foot parchment scroll. For two centuries, the scroll was lost. In the 1660s it mysteriously turned up in Pau, a town in the Pyrenees, far from Paris. A copy sent to the royal library in Paris was ignored until the 1740s, when it rekindled interest in the original, which was finally printed in the 1860s. Even today, scholars have been slow to recognize Guillaume’s pioneering detective work, apparently skeptical that true detective work was possible in an age of torture, or that there was in fact any mystery about who killed Louis.

Reading about the scroll in historical accounts and suspecting that scholars had not “gotten” Guillaume, I sought out the 1860s transcript copied from the original. Amazed by the everyday detail about ordinary Parisians like Jacquette, I was also astonished by Guillaume’s masterly inquiry, which reads like a modern police procedural. I decided to travel to Pau to examine the scroll at the archive. The transcript was complete and accurate and included a 35-line gap, perhaps left open for a deposition yet to come.

Reading Jacquette’s testimony—this time, on the tattered parchment scroll, in small, neat script now faded from black to brown—made the events of that chilly November night more than 600 years ago seem all the more real and astounding.

Much has changed in crime detection since Guillaume hurried to the scene of the horrible murder that Jacquette saw out her window. Since the rise of police detectives in the 19th century, and their modern fictional counterparts—from Arthur Conan Doyle’s Sherlock Holmes to Chandler’s Philip Marlowe and beyond—we think we know who detectives are and what they do. But the story of how a brave and uncompromising police chief of Paris solved the crime of the century is a story worthy of Chandler, and yet it’s torn from the pages of medieval history. And as a true-crime tale, it’s far stranger than fiction.