“The Deadly Safe House”- Solution
He left his overcoat at the safe house.
Captain Cromwell was on the take from the mob and, for a price, agreed to kill Fischel. It was a simple job to get the witness to open the safe house door to a police captain he already knew.
As soon as Cromwell left the murder scene, he realized that he’d forgotten his overcoat on a hook just inside the door. He couldn’t retrieve it because the door locked automatically and he had no key.
On the drive back home, Morelli remembered that Captain Cromwell had arrived at the safe house in a rumpled suit, but left carrying an overcoat. This coat undoubtedly belonged to the captain, since he later took his notepad out of the pocket.