Everyone leaves cells hairs and a trace of themselves. It’s there. Perhaps difficult to find, but it’s there.
Also casings and in particular the firing pin leaves a very unique imprint on each casing. There are databases to identify as they appear under an electron microscope. I think they will track the gun and find who did this.
In 1973 my dear friend Dawn Swan Magyar was kidnapped and murdered. Found a couple months later, there was also scant evidence. It took a lot of very diligent work and tracking a gun across the country and back to finally locate the ______ that raped and killed her. The gun was located in the river 35 miles away. At a different time her wallet was found.
That investigation was done manually. The technology has advanced by lightyears. Dawn didn’t know what a computer was in 1973.
I’ll find a link.
https://wikivisually.com/wiki/Murder_of_Dawn_Magyar
But just after her killer was convicted a group of us took some of the men and women involved to dinner. Dawn was truly an angel. Married 3 years she had a one year old son who is about 46 now. He never knew his lovely kind warm loving mom. I did and helped search for her. Her family farmed less than a mile from ours. If not for a strong belief in God, that family would have fallen apart.
It may take time. But they will find the person who did this. I’m pretty confident if it.