In Chapter 50, Geyer travels to Toronto to continue his search for clues about Holmes's past. He and a Toronto detective find that Holmes and his entourage had stayed there, again in three different hotels, each party not knowing about the other. As Geyer contemplates what drives Holmes, he receives a valid tip from a man who thinks Holmes rented the house next door to him. Geyer and the other detective go to talk to the man, who remembers Holmes borrowing a shovel from him, Holmes returning the shovel the next day, a trunk being carried away from the house the next day, and not seeing Holmes ever again afterwards. The detectives immediately go to the house next door, and the current resident leads them to the cellar. There the detectives find what is left of Alice and Nellie Pietzel buried in the cellar. Carrie Pietzel travels to Toronto to identify her dead children. The police conclude that Holmes had made a big trunk into a gas chamber to kill the girls. Geyer remains hopeful that somehow, Howard was left alive in Indianapolis.
Chapter 51 switches back to Holmes in the Philadelphia prison. Despite Geyer's efforts to keep the news from him, Holmes reads about Geyer discovering the bodies. Before being taken to the District Attorney's office for questioning later that day, Holmes, in his ongoing memoir, concocts a story about Minnie and a man named Hatch murdering the children. He then refuses to talk at the DA's office. Later, back at prison, Holmes hooks up with a journalist who wants to publish his memoir.
Geyer continues his search for Howard Pietzel, returning to Indianapolis in Chapter 52. At first, Indianapolis offers no further clues as to what happened to Howard. Meanwhile, the Chicago police decide to investigate Holmes's former Worlds' Fair Hotel. They find much evidence and conclude that Holmes had murdered many, mostly young women, in the hotel. During the investigation, the hotel burns to the ground one night, destroying whatever Chicago police had yet to uncover. Back in Indianapolis, Geyer's search leads him to the only area of the city he hadn't yet covered: Irvington. There Geyer finally discovers Howard Pietzel's remains, along with evidence and witnesses that place Holmes as the killer.
The last chapter in this group of chapters sees Holmes indicted in 1895 in many cities for several murders to ensure that he is convicted in at least one state for murder. His book is published.
These chapters provide closure concerning Holmes, although Holmes never once takes responsibility for the murders he committed. Additionally, Holmes continues to attempt to manipulate the truth and play games with peoples' lives, even while in prison. At this point, the character of Holmes truly epitomizes Larson's theme of evil.
In the midst of Chicago's embarrassment for not discovering Holmes's activity, irony is revealed: The chief of police, a former lawyer, had been Holmes's legal representative for a time while he was in Chicago.
The image of fire repeats in these final chapters, as Holmes's horrific history comes to light. Appropriately, a fire in the building that was once the World's Fair Hotel in Chicago prevents Chicago police from collecting further evidence on Holmes.
Part IV ends with a Chicago newspaper quote about Holmes. The passage implies that Holmes is so evil, no writer could possibly make him up. These lines remind readers that the book is literary nonfiction based on real people. In fact, Larson creates such a character in the person of the antagonist. By using this quote, Larson makes his book seem more like nonfiction than fiction, something he accomplishes strategically throughout the book.