New to this thread. Never heard about it before now. Thinking most of the links are gone now, I didn't go back to read. Interesting case that needs to be solved. Serial killer? Since DNA has progressed so much in the last few years, maybe they could recheck the letter, stamp and envelope? Also, there have been known mistakes in testing bones. Maybe they could recheck some of those found?
I did a search based on your link and found some articles that seem to describe the case.
From Wikipedia,
'In 2018, two cars were raised from
Benbrook Lake because they were thought to have a connection to the case.
[15] These efforts however yielded no results.'
Three missing girls. A family’s hopes. And this week, a long-awaited dive into a lake
What Happened to the Fort Worth Trio? – Things We Find in the Dark – Medium
Fort Worth Missing Trio - Wikipedia