You will notice that during cross examination of the lead detective, he admits not following up on eye witnesses who wanted to talk to an officer about evidence of an intruder. One of the witnesses called shortly after the murders and reported a man fitting the same description that Darlie gave earlier. They never checked it out! The links below will take you to the place in the transcript.
Vol 42 Page 4067 - Black car leaves as officers arrive Vol 42 Page 4074 - Suspicious black car seen earlier Vol 42 Page 4151 - Sighting of a man fitting description given by Darlie