In the book there definitely are cover-ups, >!Hale was mostly pretty careful to cover up his tracks, never going through the murders himself and often killing off co-conspirators he'd hired in what looked like routine accidents or paying off private eyes and witnesses to obfuscate the investigation. It took a while before Tom White and his men were able to detangle Hale's "plots within plots", it's not like they were just killing the Osages in public in broad daylight.!< But otherwise you're right, after J Edgar Hoover thought the FBI had done enough to make themselves look good they walked away and let everything else fade away to time.