See-n-Solve

The correct answer is Rob Sayers.

Explanation

Amy had hinted about a clue on the floor, and Harry looked and looked.

"I don't see anything," he whispered. "The shop just opened, and the floor's pretty clean."

"Too clean," Amy replied. "If you're planing a piece of wood, wouldn't you expect to leave a few shavings?"

Harry looked again. "You're right. There's nothing on the floor under Rob's station. But maybe he swept them up."

"With what? Not with the broom. It's got cobwebs left over from the weekend. No, Rob came in, went right in to confront Jensen, and killed him. When he ran out, he grabbed his plane and a piece of wood. But he didn't have time to do any work."

Rob Sayers couldn't hear their whispers. But he followed their gaze to the floor and slowly realized what they weren't looking at. When he made a break for the door, Harry was there ahead of him, drawing his service revolver from his jacket.