The correct answer is Tommy O'Dwyer.
"First, let me explain the tape."
Amy opened the alley door and stuck the tape over the latch. "This allowed the robber to get inside the building."
Harry looked puzzled. "The robber didn't need to get in. All the suspects were already inside."
"I'm talking about after the robbery. Santa came back into the basement."
"Why would he do that? The basement doesn't lead anywhere except back up to the gymnasium."
"He did it so that he could change back into his underwear and hit himself on the head. It was Tommy O'Dwyer. There was no attacker. He must have hidden the stolen items somewhere nearby."
"What about Clark and the oy mot?"
"Look at Tommy's tattoo. He was standing by the bathroom sink when Clark saw him. Clark saw the tattoo reflected in the mirror. 'Tommy O.' Read it backwards and you have it. Oymmot.'