This post is based on Eric Jager’s book, “Blood Royal” (Little, Brown and Co.)

Over the two days following the murder, Guillaume’s officers collected several dozen depositions from shopkeepers, housewives and other ordinary Parisians. A few had witnessed the crime itself, while others had unknowingly supplied the assassins with goods, and still others had seen the killers fleeing the crime scene through the city streets.

Guillaume’s officers had also collected physical evidence, including kitchen wares and other items from the rental house the assassins had used as a hideout, as well as caltrops — clusters of sharp metal spikes — that the fleeing killers had thrown behind them to delay or cripple pursuers.

An alert barber’s apprentice from a side street near the Rue Saint-Martin, a half-mile or so to the west of the crime scene, even recalled some details of the fleeing killers’ clothing that provided a very useful clue.

By Friday morning, the second day after the murder, Guillaume had reached a tentative — but very troubling — conclusion. The conspiracy to murder Louis, he now suspected, reached into some very high places.

His suspicions might put him in grave danger if he shared them beyond the Châtelet. Attempts on the lives of law officers were not unknown. In 1368, a royal sergeant had been assassinated while investigating a criminal case. Whatever Guillaume did next, he had to do with extreme care.

On the night of the murder, Guillaume had reported his initial findings to the royal council. And on Friday the council summoned him again to give a follow-up report.

The council — including the king’s uncles and cousins and other great lords of France but excluding the insane king himself — was to meet that day at the royal palace, the Hotel St. Pol, which fronted the river on the Right Bank near the city’s east wall.

When Guillaume arrived at the palace and entered the council chamber, all the lords turned to him, and one of them asked:

“What have you discovered, Provost, about the late duke’s death?”

Guillaume replied that he had been investigating the matter “with the greatest possible diligence” but that so far he had been unable to learn the truth.

This was not what the lords — most of them, that is — wanted to hear. They wanted results. They wanted to know who had killed Louis, and they wanted to know now.

But Guillaume was not finished.

“However, if my lords would allow me to enter the houses of the king’s servants, including my lords’ own residences, perhaps I may discover the truth about the perpetrators, or at least their accomplices.”

There was silence in the room as these words sank in.

Had the provost really mentioned “my lords” in the same breath as “the perpetrators” and “their accomplices”? And was he really proposing to search their own sacrosanct and inviolable homes? To ransack their bedchambers and their storerooms, their cellars and their stables, as though the royal dukes were ordinary suspects in an everyday criminal case? This was unprecedented — under the law, no one could search a prince’s house or seize property or persons there “without the permission of the lord.”

Just a month earlier, Tignonville had hanged the two self-professed clerics amid protests from the university, a dispute that still lingered in the air. What else would the impertinent provost now do? Attempt to arraign the lords of France before his tribunal? Question and depose them like ordinary citizens? Interrogate them under torture?

The lords pondered the matter, and Guillaume waited for an answer, knowing that he had taken a terrific gamble. All the lords had to do was refuse, and that would be the end of it — his investigation would be over; he might even be summarily sacked, and the crime would remain officially unsolved.

University men were also Church employees who enjoyed “benefit of clergy” — access to a separate court system with more lenient judges and often lighter sentences. The Church had at first washed its hands of the two men whom Guillaume had prosecuted for theft and murder. “Sham clerics,” ineligible for the church courts, they had been tried, convicted and hanged in a lay court. But then the Church reversed itself, reclaiming the men as its own and lodging a formal complaint against Guillaume.

But the great lords of France were in another category altogether, virtually above the law because of the “blood royal” they shared with the king. They enjoyed a special immunity from search and seizure, and even from prosecution.

Raymond Chandler famously likened the modern detective to a courageous knight errant searching for the hidden truth of a crime. Guillaume was the real thing, the original knight-sleuth — a brilliant and fearless gumshoe of the cobblestone age.

Guillaume’s field experience as a knight may have fortified him against fear. In the 1390s, he had helped lead a military expedition in central France to crush the robbers and brigands terrorizing the region. And as a diplomat he had traveled Europe to meet with princes, kings, and even the Avignon pope. In the case of the two hanged clerics, he was defying the Church, which claimed to speak for God. So his challenge to the lords of France is not so surprising. “There is no shame in doing justice,” Guillaume had once written. And he seemed to have no fear in doing justice either.

In daring the lords of France to open their doors to his investigation, Guillaume was not only doing something nearly unprecedented. He was taking the greatest gamble of his life and career in the pursuit of justice.

As Guillaume waited for an answer from the lords, everything hung in the balance — his reputation and his job, his personal safety and that of his family, the outcome of the momentous murder case, and perhaps the fate of France, since what he now suspected could plunge the nation into civil war. He waited. He would have an answer.

(Excerpts from BLOOD ROYAL, copyright Eric Jager, courtesy of Little, Brown and Company.)