This is an excerpt from “Blood Royal,” by Eric Jager, courtesy of Little, Brown and Co.

On the night of November 23, 1407, shortly after Louis of Orleans, brother of the French king, was hacked to death in the street by a gang of assassins, a messenger arrived at the home of Guillaume de Tignonville, Provost of Paris, in the Rue Béthisy, nearly a mile to the west of where Louis had been struck down.

Guillaume was a knight, a diplomat and a valued royal adviser. For the past six years, he had been Provost of Paris, the king’s chief law enforcement officer, responsible for keeping order in Paris and solving crimes. He lived near the Louvre in an imposing stone mansion with his wife, Alix, and their daughter. The news he got that night would plunge Guillaume into one of the darkest mysteries ever to astound France — and into great personal danger that would cause him to fear for his family as well.

On hearing of Louis’s death, Guillaume immediately summoned his lieutenant and a large body of officers, all armed and carrying torches. He led them through the sleeping city to the Rue Vieille du Temple, a street that was now wide awake, with neighbors gawking at the muddy pavement still streaked with blood, and smoke hanging in the air from a fire apparently set by the assasins before fleeing their hideout.

Louis’s body had already been carried into a nearby house, where Guillaume found it laid out on a table and covered with a piece of black damask. The head and face were so horribly mangled that Guillaume, who knew Louis well, may have hardly recognized the victim at first. His report describes what he saw:

Item, two wounds to the head, one running from the left eye to the right ear, and the other running from the left ear almost to the right ear. And the wounds were of such a kind, and so enormous, that the head was all sliced up and the entire brain protruded.

Item, his left hand was severed completely from the arm between the thumb and the wrist.

Item, his right arm was broken so that the master bone protruded below the elbow, and the arm also had a great wound in it.

Autopsies were rare at this time; the first recorded medical dissection in Paris had taken place in that same year, 1407. Yet Guillaume’s report is anatomically precise, even clinical. As a knight, he knew the terrible wounds that various kinds of weapons could inflict. Had he also studied medicine? Or had he simply acquired a coroner’s observant eye during his years as Provost? His headquarters, at the Châtelet, had a morgue where bodies found in the city were brought for identification, packed in salt and straw to preserve them. He was used to examining corpses to determine the cause of death.

As Guillaume studied the body, the cause of death was just one question on his mind. Who had murdered Louis? And why? There were many possible suspects and motives. A knight named Albert de Chauny, for instance, was known to hate Louis for having taken his wife as his mistress. And other nobles loathed the king’s brother for personal or political reasons. Not even the insane king could be ruled out, having once chased his brother with a sword during a fit of madness.

Guillaume was also shown another body — that of a squire who had been killed while defending Louis. “It appeared,” Guillaume noted, careful not to jump to conclusions, that the wounds on both victims had caused their deaths. Questioning the people on the scene, Guillaume tentatively concluded that “the duke of Orleans and his people were killed and murdered in the said street in front of the house while passing along the street, and that those who had done this thing had fled, leaving the body all dead in the mud of the street.”

The methodical Provost next notified the lords of France, who would call an emergency meeting of the royal council where they would hear Guillaume’s initial report. He also ordered Paris to be locked down, posting guards in the streets to prevent panic and closing all the city gates to stop the killers from escaping. He ordered a search of the assassins’ hideout and an inventory of everything in it. He delegated his officers to take depositions from people living near the crime scene. And he sent criers into the streets ordering innkeepers to send lists of their guests to the Châtelet for a census of city visitors.

Was Guillaume a typical investigator — or somewhat ahead of his time, as far as police work went? Claude Gauvard, a highly respected expert on medieval law, writes that Guillaume conducted his inquiry with “a remarkable legal and scientific rigor” — suggesting the Provost’s unusual diligence for his time. As Guillaume’s report shows, he rapidly mobilized the scores of officers and clerics at his command to mount a citywide manhunt and a dragnet for evidence in order to break the case as quickly as possible.

Yet there is one medieval crime-solving method that Guillaume evidently refrained from using in this case, despite the enormity of the crime and the urgency of the situation, and despite the fact that the law allowed and even prescribed it: judicial torture — the use of coercion and pain to extract evidence from witnesses or suspected felons.

Next time: An eyewitness to the murder is deposed.