It’s truly a proverbial “needle in the haystack.” Mountains of evidence don’t always equate to or prove to be beneficial. Many times they prove to be frustrating lessons in futility. It’s not that the killer didn’t mess up and leave evidence. It’s weather or not they were able to collect and recognize it prior to contamination. Instead of searching for the killer, they should be actively focusing on the tried and true art of deduction.
Go through the neighboring apartments AGAIN, specifically those (all and any) with a direct line of sight on that home. COLD Knock on doors at 03:00, speak with the occupants who answer. Ask all the same list of questions. Wear body cams to record every interaction and review it all (more closely) later with the feds. Are the occupants of the apartments awake at that hour? Asleep? Has anyone visited any tenant in said apartments or frat houses (as a guest - possibly from out of state) during the time frame in question? Perhaps a family member, boyfriend, or friend. Is there an insomniac among them? Anyone with prior military experience (or current) Specific to the USMC or USN? DD or HD. ETC.