Philip Vannatter was such a moron. Never believed a word he said.
Fuhrman described this dynamic in detail. He was disappointed they would not stay on the case after doing the initial footwork, but he was professional about it and did his job if his account and that of his boss are to be believed.
He went out of his way to mention the bloody thumb print and said it in his notes too.
Any competent detective would have zeroed in on that thumbprint as the first order of business. It sounded like Vannatter just put the handwritten notes in a folder and didn’t read through them again.