These Shallow Graves by Jennifer Donnelly

killed my father didn’t match those of the other five bullets still inside his revolver.”
    â€œWhich apparently didn’t trouble Koehler, as Oscar told us. Or the cops,” Eddie said.
    â€œDoes it trouble you?” Jo asked.
    Eddie nodded. “It does,” he said. “Sportsmen tend to have favorites. Favorite fishing rods. Favorite lures. Favorite gunmakers and ammunition. Why would your father switch from one make of bullet to another?” He looked at his notepad again. “Koehler didn’t think the different markings were important. The cops didn’t even record them in their report. But Oscar thinks the presence of a different casing means the killer used a different gun.”
    â€œBut if that’s so, how did the casing from the killer’s gun get inside my father’s revolver?” Jo countered.
    â€œGood question,” Eddie said, frowning. He flipped a page in his notebook. “Oscar said the casing was marked UMC .38 S & W and the unfired bullets were marked W.R.A. Co. .38 LONG.”
    Jo drew a sharp breath. She knew that last mark. She’d seen it very recently.
    Eddie’s eyes darted to her face. “What is it?” he asked.
    â€œI just remembered something,” she said. “I should’ve thought of it back at the morgue, but I was too upset. The day of my father’s funeral, I went to his study and found a bullet. It was on the floor, tangled up in the carpet’s fringe. At the time, I thought Papa might’ve left some bullets loose on the desk while he was cleaning his gun and knocked them off after he shot himself, as he fell to the floor. And that someone—Theakston, maybe—had kicked one across the room.”
    Eddie sat forward in his chair, his gaze intense. “Do you remember the mark on that bullet?”
    â€œYes. It was W.R.A. Co. .38 LONG,” she said, her gaze equally intense. “Eddie, what if—”
    â€œThe killer fired the lethal shot from his gun, then found your father’s loaded revolver.” Eddie said.
    â€œHe replaced one of the bullets in the chamber with the spent casing, and then put the revolver into my father’s hand! It makes sense, doesn’t it?” Jo asked, excited that they’d come up with a plausible explanation for the presence of two different bullets.
    But instead of echoing her excitement, Eddie frowned again.
    â€œWhat?” Jo asked.
    â€œHow could the killer be smart enough to replace a bullet with the spent casing, and then be stupid enough to drop the bullet on the floor?” he asked.
    â€œMaybe something spooked him. Maybe he heard footsteps or shouting,” Jo offered.
    Eddie nodded but didn’t seem convinced. He turned back to his notes.
    â€œWhen Buckley declared your father dead, your mother—who was still outside the study—became extremely distraught. Mrs. Nelson took her to her room. Theakston told Miss Klopp to fetch Phillip Montfort, and the other two maids to fix a pot of coffee. Buckley stopped him. He said he needed one girl to go to the station house to tell his captain what had happened. Miss McManus went. Miss Schmidt went to the kitchen. They all went back to their rooms first, though, to get dressed.”
    Eddie paused to take a sip of his coffee, then continued.
    â€œBuckley sent Dolan for Dr. Koehler. He then asked Theakston to show him all possible exits from the house. They tested the servants’ door and the door that leads from your kitchen to your back garden. Both were locked. Theakston had run out of the front door to fetch Dolan but said he had to unlock it first. There are four master keys to the doors. Buckley stated that he was unable to search for them right away, because the coroner arrived, followed by the police captain and Phillip Montfort, and he had to brief them, but he confirmed the keys’ whereabouts before he left. One was your father’s and was found in his